Extended low-frequency non-separable transform (LFNST) designs with worst-case complexity handling

ABSTRACT

A video decoder can be configured to determine a number of allowed non-zero coefficients for a block of video data based on a size of the block; obtain a set of dequantized coefficients for the block, wherein the set of dequantized coefficients comprises a first subset of dequantized coefficients that includes non-zero dequantized coefficients and a second subset of dequantized coefficients that includes all zero coefficients, wherein a number of coefficients in the first subset of dequantized coefficients is equal to the number of allowed non-zero coefficients for the block of video data; apply an inverse low-frequency non-separable transform (LFNST) to the first subset of dequantized coefficients to determine a first intermediate subset of coefficients; and apply an inverse separable transform to the first intermediate subset of coefficients and at least a portion of the second subset of coefficients to determine a block of reconstructed residual values.

This application claims the benefit of U.S. Provisional PatentApplication 63/086,888, filed 2 Oct. 2020, the entire content of whichis incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to video encoding and video decoding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, tablet computers, e-book readers, digitalcameras, digital recording devices, digital media players, video gamingdevices, video game consoles, cellular or satellite radio telephones,so-called “smart phones,” video teleconferencing devices, videostreaming devices, and the like. Digital video devices implement videocoding techniques, such as those described in the standards defined byMPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced VideoCoding (AVC), ITU-T H.265/High Efficiency Video Coding (HEVC), andextensions of such standards. The video devices may transmit, receive,encode, decode, and/or store digital video information more efficientlyby implementing such video coding techniques.

Video coding techniques include spatial (intra-picture) predictionand/or temporal (inter-picture) prediction to reduce or removeredundancy inherent in video sequences. For block-based video coding, avideo slice (e.g., a video picture or a portion of a video picture) maybe partitioned into video blocks, which may also be referred to ascoding tree units (CTUs), coding units (CUs) and/or coding nodes. Videoblocks in an intra-coded (I) slice of a picture are encoded usingspatial prediction with respect to reference samples in neighboringblocks in the same picture. Video blocks in an inter-coded (P or B)slice of a picture may use spatial prediction with respect to referencesamples in neighboring blocks in the same picture or temporal predictionwith respect to reference samples in other reference pictures. Picturesmay be referred to as frames, and reference pictures may be referred toas reference frames.

SUMMARY

The techniques of this disclosure are related to transform coding. Morespecifically, this disclosure describes various low-frequencynon-separable transform (LFNST) designs that may improve codingefficiency in Versatile Video Coding (VVC/H.266). The techniques of thisdisclosure may also be used in other advanced video codecs includingextensions of HEVC and the next generation of video coding standards.

For hardware implementations, the worst-case number of multiplication(per-transform coefficient) may be an important complexity criterion.One simple technique for reducing the worst-case number ofmultiplications is to normatively zero-out some of the transformcoefficients and have a maximum number of non-zero transformcoefficients. In this context, a zeroed-out transform coefficient mustbe set equal to zero. In contrast, an allowed non-zero transformcoefficient may be either zero or non-zero. In this regard, the numberof allowed non-zero transform coefficients may represent a maximumpossible number of non-zero transform coefficients, and an actual numberof non-zero transform coefficients may be less than the maximum.Conversely, a number of zeroed-out transform coefficients may representa minimum number of transform coefficients that equal zero, and anactual number of transform coefficients that equal zero may be greaterthan the minimum.

The zeroing-out process may reduce coding quality and thus may beundesirable unless required to meet a certain worst-case number ofmultiplications. In VVC, these criteria are hardwired for TU sizes,which in some coding scenarios, results in zeroing out more transformcoefficients than necessary to meet worst-case number of multiplicationscriteria. To address this shortcoming, this disclosure describestechniques for determining a number of allowed non-zero transformcoefficients as a function of the transform dimensions and thedimensions of the transform matrix (including the number of “supportsamples”). By determining a number of allowed non-zero coefficients fora block of video data based on a size of the block, a video decoderoperating according to the techniques of this disclosure may avoidunnecessarily zeroing out coefficients when not needed, while stillcomplying with worst-case multiplication criterion.

According to one example, a method of decoding video data includesdetermining a number of allowed non-zero coefficients for a block ofvideo data based on a size of the block; obtaining a set of dequantizedcoefficients for the block of video data, wherein the set of dequantizedcoefficients comprises a first subset of dequantized coefficients thatincludes non-zero dequantized coefficients and a second subset ofdequantized coefficients that includes all zero coefficients, wherein anumber of coefficients in the first subset of dequantized coefficientsis equal to the number of allowed non-zero coefficients for the block ofvideo data; applying an inverse low-frequency non-separable transform(LFNST) to the first subset of dequantized coefficients to determine afirst intermediate subset of coefficients; and applying an inverseseparable transform to the first intermediate subset of coefficients andat least a portion of the second subset of coefficients to determine ablock of reconstructed residual values.

According to another example, a device for decoding video data includesa memory configured to store video data and one or more processorsimplemented in circuitry and configured to determine a number of allowednon-zero coefficients for a block of video data based on a size of theblock; obtain a set of dequantized coefficients for the block of videodata, wherein the set of dequantized coefficients comprises a firstsubset of dequantized coefficients that includes non-zero dequantizedcoefficients and a second subset of dequantized coefficients thatincludes all zero coefficients, wherein a number of coefficients in thefirst subset of dequantized coefficients is equal to the number ofallowed non-zero coefficients for the block of video data; apply aninverse low-frequency non-separable transform (LFNST) to the firstsubset of dequantized coefficients to determine a first intermediatesubset of coefficients; and apply an inverse separable transform to thefirst intermediate subset of coefficients and at least a portion of thesecond subset of coefficients to determine a block of reconstructedresidual values.

According to another example, a computer-readable storage medium storesinstructions that when executed by one or more processors cause the oneor more processors to determine a number of allowed non-zerocoefficients for a block of video data based on a size of the block;obtain a set of dequantized coefficients for the block of video data,wherein the set of dequantized coefficients comprises a first subset ofdequantized coefficients that includes non-zero dequantized coefficientsand a second subset of dequantized coefficients that includes all zerocoefficients, wherein a number of coefficients in the first subset ofdequantized coefficients is equal to the number of allowed non-zerocoefficients for the block of video data; apply an inverse low-frequencynon-separable transform (LFNST) to the first subset of dequantizedcoefficients to determine a first intermediate subset of coefficients;and apply an inverse separable transform to the first intermediatesubset of coefficients and at least a portion of the second subset ofcoefficients to determine a block of reconstructed residual values.

According to another example, a device for decoding video data includesmeans for determining a number of allowed non-zero coefficients for ablock of video data based on a size of the block; means for obtaining aset of dequantized coefficients for the block of video data, wherein theset of dequantized coefficients comprises a first subset of dequantizedcoefficients that includes non-zero dequantized coefficients and asecond subset of dequantized coefficients that includes all zerocoefficients, wherein a number of coefficients in the first subset ofdequantized coefficients is equal to the number of allowed non-zerocoefficients for the block of video data; means for applying an inverselow-frequency non-separable transform (LFNST) to the first subset ofdequantized coefficients to determine a first intermediate subset ofcoefficients; and means for applying an inverse separable transform tothe first intermediate subset of coefficients and at least a portion ofthe second subset of coefficients to determine a block of reconstructedresidual values.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system that may perform the techniques of this disclosure.

FIGS. 2A and 2B are conceptual diagrams illustrating an example quadtreebinary tree (QTBT) structure, and a corresponding coding tree unit(CTU).

FIG. 3 is a block diagram illustrating an example low-frequencynon-separable transform (LFNST) at an encoder and a decoder.

FIG. 4 is a conceptual diagram illustrating an inverse transform processwhen LFNST is used.

FIG. 5 is a conceptual diagram illustrating an 4×4 inverse LFNST used toreconstruct 16 intermediate coefficients from a list of 16 inputcoefficients.

FIG. 6 shows an illustration of 8×8 inverse LFNST used to reconstruct 48intermediate coefficients form a list of 16 input coefficients.

FIG. 7 shows an illustration of inverse LFNST process.

FIG. 8 shows an example of input reorganization based on raster-scanwith 16 decoded coefficients.

FIG. 9 shows an example of input reorganization based on raster-scanwith 64 decoded coefficients.

FIG. 10 shows an illustration of 8×8 inverse LFNST used to reconstruct64 intermediate coefficients from 16 input coefficients.

FIG. 11 shows an example illustration of 8×8 inverse LFNST used toreconstruct 64 intermediate coefficients from 64 input coefficients.

FIG. 12 is a block diagram illustrating an example video encoder thatmay perform the techniques of this disclosure.

FIG. 13 is a block diagram illustrating an example video decoder thatmay perform the techniques of this disclosure.

FIG. 14 is a flowchart illustrating an example process for encoding acurrent block in accordance with the techniques of this disclosure.

FIG. 15 is a flowchart illustrating an example process for decoding acurrent block in accordance with the techniques of this disclosure.

FIG. 16 is a flowchart illustrating an example process for decoding acurrent block in accordance with the techniques of this disclosure.

DETAILED DESCRIPTION

Video coding (e.g., video encoding and/or video decoding) typicallyinvolves predicting a block of video data from either an already codedblock of video data in the same picture (e.g., intra prediction) or analready coded block of video data in a different picture (e.g., interprediction). In some instances, the video encoder also calculatesresidual data by comparing the prediction block to the original block.Thus, the residual data represents a difference between the predictionblock and the original block. To reduce the number of bits needed tosignal the residual data, the video encoder transforms and quantizes theresidual data and signals the transformed and quantized residual data inthe encoded bitstream. The compression achieved by the transform andquantization processes may be lossy, meaning that transform andquantization processes may introduce distortion into the decoded videodata.

A video decoder decodes and adds the residual data to the predictionblock to produce a reconstructed video block that matches the originalvideo block more closely than the prediction block alone. Due to theloss introduced by the transforming and quantizing of the residual data,the first reconstructed block may have distortion or artifacts. Onecommon type of artifact or distortion is referred to as blockiness,where the boundaries of the blocks used to code the video data arevisible.

To further improve the quality of decoded video, a video decoder canperform one or more filtering operations on the reconstructed videoblocks. Examples of these filtering operations include deblockingfiltering, sample adaptive offset (SAO) filtering, and adaptive loopfiltering (ALF). Parameters for these filtering operations may either bedetermined by a video encoder and explicitly signaled in the encodedvideo bitstream or may be implicitly determined by a video decoderwithout needing the parameters to be explicitly signaled in the encodedvideo bitstream.

The techniques of this disclosure are related to transform coding. Morespecifically, this disclosure describes various low-frequencynon-separable transform (LFNST) designs that may improve codingefficiency in Versatile Video Coding (VVC/H.266). The techniques of thisdisclosure may also be used in other advanced video codecs includingextensions of HEVC and the next generation of video coding standards.

When implementing LFNST, a video decoder performs a coefficient decodingstep to obtain a list of 2-D dequantized coefficient. The decoderapplies an inverse LFNST to reconstruct a subset of coefficients. Then,the video decoder uses an inverse separable transform (e.g., DCT-2) toreconstruct residuals in a 2-D block/array. In some examples, a videodecoder may use larger transform matrices (increase in number of supportsamples), and accordingly, a larger number of transform sets andcandidates to allow for matrices optimized for more intra predictionmodes.

For hardware implementations, the worst-case number of multiplication(per-transform coefficient) may be an important complexity criterion.One simple technique for reducing the worst-case number ofmultiplications is to normatively zero-out some of the transformcoefficients and have a maximum number of non-zero transformcoefficients. In this context, a zeroed-out transform coefficient mustbe set equal to zero. In contrast, an allowed non-zero transformcoefficient may be either zero or non-zero. In this regard, the numberof allowed non-zero transform coefficients may represent a maximumpossible number of non-zero transform coefficients, and an actual numberof non-zero transform coefficients may be less than the maximum.Conversely, a number of zeroed-out transform coefficients may representa minimum number of transform coefficients that equal zero, and anactual number of transform coefficients that equal zero may be greaterthan the minimum.

The zeroing-out process may reduce coding quality and thus may beundesirable unless required to meet a certain worst-case number ofmultiplications. In VVC, these criteria are hardwired for TU sizes,which in some coding scenarios, results in zeroing out more transformcoefficients than necessary to meet worst-case number of multiplicationscriteria. To address this shortcoming, this disclosure describestechniques for determining a number of allowed non-zero transformcoefficients as a function of the transform dimensions and thedimensions of the transform matrix (including the number of “supportsamples”). By determining a number of allowed non-zero coefficients fora block of video data based on a size of the block, a video decoderoperating according to the techniques of this disclosure may avoidunnecessarily zeroing out coefficients when not needed, while stillcomplying with worst-case multiplication criterion.

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 100 that may perform the techniques of this disclosure.The techniques of this disclosure are generally directed to coding(encoding and/or decoding) video data. In general, video data includesany data for processing a video. Thus, video data may include raw,unencoded video, encoded video, decoded (e.g., reconstructed) video, andvideo metadata, such as signaling data.

As shown in FIG. 1 , system 100 includes a source device 102 thatprovides encoded video data to be decoded and displayed by a destinationdevice 116, in this example. In particular, source device 102 providesthe video data to destination device 116 via a computer-readable medium110. Each of source device 102 and destination device 116 may be any oneor more of a wide range of devices, including a desktop computer, anotebook (i.e., laptop) computer, a mobile device, a tablet computer, aset-top box, a telephone handset such as a smartphone, a television, acamera, a display device, a digital media player, a video gamingconsole, a video streaming device, a broadcast receiver device, or thelike. In some cases, source device 102 and destination device 116 may beequipped for wireless communication, and thus may be referred to aswireless communication devices.

In the example of FIG. 1 , source device 102 includes video source 104,memory 106, video encoder 200, and output interface 108. Destinationdevice 116 includes input interface 122, video decoder 300, memory 120,and display device 118. In accordance with this disclosure, videoencoder 200 of source device 102 and video decoder 300 of destinationdevice 116 may be configured to apply the LFNST techniques describedherein. Thus, source device 102 represents an example of a videoencoding device, while destination device 116 represents an example of avideo decoding device. In other examples, a source device and adestination device may include other components or arrangements. Forexample, source device 102 may receive video data from an external videosource, such as an external camera. Likewise, destination device 116 mayinterface with an external display device, rather than include anintegrated display device.

System 100 as shown in FIG. 1 is merely one example. In general, anydigital video encoding and/or decoding device may perform the LFNSTtechniques described herein. Source device 102 and destination device116 are merely examples of such coding devices in which source device102 generates coded video data for transmission to destination device116. This disclosure refers to a “coding” device as a device thatperforms coding (encoding and/or decoding) of data. Thus, video encoder200 and video decoder 300 represent examples of coding devices, inparticular, a video encoder and a video decoder, respectively. In someexamples, source device 102 and destination device 116 may operate in asubstantially symmetrical manner such that each of source device 102 anddestination device 116 includes video encoding and decoding components.Hence, system 100 may support one-way or two-way video transmissionbetween source device 102 and destination device 116, e.g., for videostreaming, video playback, video broadcasting, or video telephony.

In general, video source 104 represents a source of video data (i.e.,raw, unencoded video data) and provides a sequential series of pictures(also referred to as “frames”) of the video data to video encoder 200,which encodes data for the pictures. Video source 104 of source device102 may include a video capture device, such as a video camera, a videoarchive containing previously captured raw video, and/or a video feedinterface to receive video from a video content provider. As a furtheralternative, video source 104 may generate computer graphics-based dataas the source video, or a combination of live video, archived video, andcomputer-generated video. In each case, video encoder 200 encodes thecaptured, pre-captured, or computer-generated video data. Video encoder200 may rearrange the pictures from the received order (sometimesreferred to as “display order”) into a coding order for coding. Videoencoder 200 may generate a bitstream including encoded video data.Source device 102 may then output the encoded video data via outputinterface 108 onto computer-readable medium 110 for reception and/orretrieval by, e.g., input interface 122 of destination device 116.

Memory 106 of source device 102 and memory 120 of destination device 116represent general purpose memories. In some examples, memories 106, 120may store raw video data, e.g., raw video from video source 104 and raw,decoded video data from video decoder 300. Additionally oralternatively, memories 106, 120 may store software instructionsexecutable by, e.g., video encoder 200 and video decoder 300,respectively. Although memory 106 and memory 120 are shown separatelyfrom video encoder 200 and video decoder 300 in this example, it shouldbe understood that video encoder 200 and video decoder 300 may alsoinclude internal memories for functionally similar or equivalentpurposes. Furthermore, memories 106, 120 may store encoded video data,e.g., output from video encoder 200 and input to video decoder 300. Insome examples, portions of memories 106, 120 may be allocated as one ormore video buffers, e.g., to store raw, decoded, and/or encoded videodata.

Computer-readable medium 110 may represent any type of medium or devicecapable of transporting the encoded video data from source device 102 todestination device 116. In one example, computer-readable medium 110represents a communication medium to enable source device 102 totransmit encoded video data directly to destination device 116 inreal-time, e.g., via a radio frequency network or computer-basednetwork. Output interface 108 may modulate a transmission signalincluding the encoded video data, and input interface 122 may demodulatethe received transmission signal, according to a communication standard,such as a wireless communication standard. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 102 to destination device 116.

In some examples, source device 102 may output encoded data from outputinterface 108 to storage device 112. Similarly, destination device 116may access encoded data from storage device 112 via input interface 122.Storage device 112 may include any of a variety of distributed orlocally accessed data storage media such as a hard drive, Blu-ray discs,DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or anyother suitable digital storage media for storing encoded video data.

In some examples, source device 102 may output encoded video data tofile server 114 or another intermediate storage device that may storethe encoded video data generated by source device 102. Destinationdevice 116 may access stored video data from file server 114 viastreaming or download.

File server 114 may be any type of server device capable of storingencoded video data and transmitting that encoded video data to thedestination device 116. File server 114 may represent a web server(e.g., for a website), a server configured to provide a file transferprotocol service (such as File Transfer Protocol (FTP) or File Deliveryover Unidirectional Transport (FLUTE) protocol), a content deliverynetwork (CDN) device, a hypertext transfer protocol (HTTP) server, aMultimedia Broadcast Multicast Service (MBMS) or Enhanced MBMS (eMBMS)server, and/or a network attached storage (NAS) device. File server 114may, additionally or alternatively, implement one or more HTTP streamingprotocols, such as Dynamic Adaptive Streaming over HTTP (DASH), HTTPLive Streaming (HLS), Real Time Streaming Protocol (RTSP), HTTP DynamicStreaming, or the like.

Destination device 116 may access encoded video data from file server114 through any standard data connection, including an Internetconnection. This may include a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., digital subscriber line (DSL),cable modem, etc.), or a combination of both that is suitable foraccessing encoded video data stored on file server 114. Input interface122 may be configured to operate according to any one or more of thevarious protocols discussed above for retrieving or receiving media datafrom file server 114, or other such protocols for retrieving media data.

Output interface 108 and input interface 122 may represent wirelesstransmitters/receivers, modems, wired networking components (e.g.,Ethernet cards), wireless communication components that operateaccording to any of a variety of IEEE 802.11 standards, or otherphysical components. In examples where output interface 108 and inputinterface 122 comprise wireless components, output interface 108 andinput interface 122 may be configured to transfer data, such as encodedvideo data, according to a cellular communication standard, such as 4G,4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In someexamples where output interface 108 comprises a wireless transmitter,output interface 108 and input interface 122 may be configured totransfer data, such as encoded video data, according to other wirelessstandards, such as an IEEE 802.11 specification, an IEEE 802.15specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. Insome examples, source device 102 and/or destination device 116 mayinclude respective system-on-a-chip (SoC) devices. For example, sourcedevice 102 may include an SoC device to perform the functionalityattributed to video encoder 200 and/or output interface 108, anddestination device 116 may include an SoC device to perform thefunctionality attributed to video decoder 300 and/or input interface122.

The techniques of this disclosure may be applied to video coding insupport of any of a variety of multimedia applications, such asover-the-air television broadcasts, cable television transmissions,satellite television transmissions, Internet streaming videotransmissions, such as dynamic adaptive streaming over HTTP (DASH),digital video that is encoded onto a data storage medium, decoding ofdigital video stored on a data storage medium, or other applications.

Input interface 122 of destination device 116 receives an encoded videobitstream from computer-readable medium 110 (e.g., a communicationmedium, storage device 112, file server 114, or the like). The encodedvideo bitstream may include signaling information defined by videoencoder 200, which is also used by video decoder 300, such as syntaxelements having values that describe characteristics and/or processingof video blocks or other coded units (e.g., slices, pictures, groups ofpictures, sequences, or the like). Display device 118 displays decodedpictures of the decoded video data to a user. Display device 118 mayrepresent any of a variety of display devices such as a liquid crystaldisplay (LCD), a plasma display, an organic light emitting diode (OLED)display, or another type of display device.

Although not shown in FIG. 1 , in some examples, video encoder 200 andvideo decoder 300 may each be integrated with an audio encoder and/oraudio decoder, and may include appropriate MUX-DEMUX units, or otherhardware and/or software, to handle multiplexed streams including bothaudio and video in a common data stream. If applicable, MUX-DEMUX unitsmay conform to the ITU H.223 multiplexer protocol, or other protocolssuch as the user datagram protocol (UDP).

Video encoder 200 and video decoder 300 each may be implemented as anyof a variety of suitable encoder and/or decoder circuitry, such as oneor more microprocessors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs), discrete logic, software, hardware, firmware or anycombinations thereof. When the techniques are implemented partially insoftware, a device may store instructions for the software in asuitable, non-transitory computer-readable medium and execute theinstructions in hardware using one or more processors to perform thetechniques of this disclosure. Each of video encoder 200 and videodecoder 300 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device. A device including video encoder 200 and/orvideo decoder 300 may comprise an integrated circuit, a microprocessor,and/or a wireless communication device, such as a cellular telephone.

Video encoder 200 and video decoder 300 may operate according to a videocoding standard, such as ITU-T H.265, also referred to as HighEfficiency Video Coding (HEVC) or extensions thereto, such as themulti-view and/or scalable video coding extensions. Alternatively, videoencoder 200 and video decoder 300 may operate according to otherproprietary or industry standards, such as ITU-T H.266, also referred toas Versatile Video Coding (VVC). A draft of the VVC standard isdescribed in Bross, et al. “Versatile Video Coding (Draft 10),” JointVideo Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG11, 18^(th) Meeting: by teleconference, 22 Jun.-1 Jul. 2020,JVET-52001-v17 (hereinafter “VVC Draft 10”). The techniques of thisdisclosure, however, are not limited to any particular coding standard.

In general, video encoder 200 and video decoder 300 may performblock-based coding of pictures. The term “block” generally refers to astructure including data to be processed (e.g., encoded, decoded, orotherwise used in the encoding and/or decoding process). For example, ablock may include a two-dimensional matrix of samples of luminanceand/or chrominance data. In general, video encoder 200 and video decoder300 may code video data represented in a YUV (e.g., Y, Cb, Cr) format.That is, rather than coding red, green, and blue (RGB) data for samplesof a picture, video encoder 200 and video decoder 300 may code luminanceand chrominance components, where the chrominance components may includeboth red hue and blue hue chrominance components. In some examples,video encoder 200 converts received RGB formatted data to a YUVrepresentation prior to encoding, and video decoder 300 converts the YUVrepresentation to the RGB format. Alternatively, pre- andpost-processing units (not shown) may perform these conversions.

This disclosure may generally refer to coding (e.g., encoding anddecoding) of pictures to include the process of encoding or decodingdata of the picture. Similarly, this disclosure may refer to coding ofblocks of a picture to include the process of encoding or decoding datafor the blocks, e.g., prediction and/or residual coding. An encodedvideo bitstream generally includes a series of values for syntaxelements representative of coding decisions (e.g., coding modes) andpartitioning of pictures into blocks. Thus, references to coding apicture or a block should generally be understood as coding values forsyntax elements forming the picture or block.

HEVC defines various blocks, including coding units (CUs), predictionunits (PUs), and transform units (TUs). According to HEVC, a video coder(such as video encoder 200) partitions a coding tree unit (CTU) into CUsaccording to a quadtree structure. That is, the video coder partitionsCTUs and CUs into four equal, non-overlapping squares, and each node ofthe quadtree has either zero or four child nodes. Nodes without childnodes may be referred to as “leaf nodes,” and CUs of such leaf nodes mayinclude one or more PUs and/or one or more TUs. The video coder mayfurther partition PUs and TUs. For example, in HEVC, a residual quadtree(RQT) represents partitioning of TUs. In HEVC, PUs representinter-prediction data, while TUs represent residual data. CUs that areintra-predicted include intra-prediction information, such as anintra-mode indication.

As another example, video encoder 200 and video decoder 300 may beconfigured to operate according to VVC. According to VVC, a video coder(such as video encoder 200) partitions a picture into a plurality ofcoding tree units (CTUs). Video encoder 200 may partition a CTUaccording to a tree structure, such as a quadtree-binary tree (QTBT)structure or Multi-Type Tree (MTT) structure. The QTBT structure removesthe concepts of multiple partition types, such as the separation betweenCUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a firstlevel partitioned according to quadtree partitioning, and a second levelpartitioned according to binary tree partitioning. A root node of theQTBT structure corresponds to a CTU. Leaf nodes of the binary treescorrespond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using aquadtree (QT) partition, a binary tree (BT) partition, and one or moretypes of triple tree (TT) (also called ternary tree (TT)) partitions. Atriple or ternary tree partition is a partition where a block is splitinto three sub-blocks. In some examples, a triple or ternary treepartition divides a block into three sub-blocks without dividing theoriginal block through the center. The partitioning types in MTT (e.g.,QT, BT, and TT), may be symmetrical or asymmetrical.

In some examples, video encoder 200 and video decoder 300 may use asingle QTBT or MTT structure to represent each of the luminance andchrominance components, while in other examples, video encoder 200 andvideo decoder 300 may use two or more QTBT or MTT structures, such asone QTBT/MTT structure for the luminance component and another QTBT/MTTstructure for both chrominance components (or two QTBT/MTT structuresfor respective chrominance components).

Video encoder 200 and video decoder 300 may be configured to usequadtree partitioning per HEVC, QTBT partitioning, MTT partitioning, orother partitioning structures. For purposes of explanation, thedescription of the techniques of this disclosure is presented withrespect to QTBT partitioning. However, it should be understood that thetechniques of this disclosure may also be applied to video codersconfigured to use quadtree partitioning, or other types of partitioningas well.

In some examples, a CTU includes a coding tree block (CTB) of lumasamples, two corresponding CTBs of chroma samples of a picture that hasthree sample arrays, or a CTB of samples of a monochrome picture or apicture that is coded using three separate color planes and syntaxstructures used to code the samples. A CTB may be an N×N block ofsamples for some value of N such that the division of a component intoCTBs is a partitioning. A component is an array or single sample fromone of the three arrays (luma and two chroma) that compose a picture in4:2:0, 4:2:2, or 4:4:4 color format or the array or a single sample ofthe array that compose a picture in monochrome format. In some examples,a coding block is an M×N block of samples for some values of M and Nsuch that a division of a CTB into coding blocks is a partitioning.

The blocks (e.g., CTUs or CUs) may be grouped in various ways in apicture. As one example, a brick may refer to a rectangular region ofCTU rows within a particular tile in a picture. A tile may be arectangular region of CTUs within a particular tile column and aparticular tile row in a picture. A tile column refers to a rectangularregion of CTUs having a height equal to the height of the picture and awidth specified by syntax elements (e.g., such as in a picture parameterset). A tile row refers to a rectangular region of CTUs having a heightspecified by syntax elements (e.g., such as in a picture parameter set)and a width equal to the width of the picture.

In some examples, a tile may be partitioned into multiple bricks, eachof which may include one or more CTU rows within the tile. A tile thatis not partitioned into multiple bricks may also be referred to as abrick. However, a brick that is a true subset of a tile may not bereferred to as a tile.

The bricks in a picture may also be arranged in a slice. A slice may bean integer number of bricks of a picture that may be exclusivelycontained in a single network abstraction layer (NAL) unit. In someexamples, a slice includes either a number of complete tiles or only aconsecutive sequence of complete bricks of one tile.

This disclosure may use “N×N” and “N by N” interchangeably to refer tothe sample dimensions of a block (such as a CU or other video block) interms of vertical and horizontal dimensions, e.g., 16×16 samples or 16by 16 samples. In general, a 16×16 CU will have 16 samples in a verticaldirection (y=16) and 16 samples in a horizontal direction (x=16).Likewise, an N×N CU generally has N samples in a vertical direction andN samples in a horizontal direction, where N represents a nonnegativeinteger value. The samples in a CU may be arranged in rows and columns.Moreover, CUs need not necessarily have the same number of samples inthe horizontal direction as in the vertical direction. For example, CUsmay comprise N×M samples, where M is not necessarily equal to N.

Video encoder 200 encodes video data for CUs representing predictionand/or residual information, and other information. The predictioninformation indicates how the CU is to be predicted in order to form aprediction block for the CU. The residual information generallyrepresents sample-by-sample differences between samples of the CU priorto encoding and the prediction block.

To predict a CU, video encoder 200 may generally form a prediction blockfor the CU through inter-prediction or intra-prediction.Inter-prediction generally refers to predicting the CU from data of apreviously coded picture, whereas intra-prediction generally refers topredicting the CU from previously coded data of the same picture. Toperform inter-prediction, video encoder 200 may generate the predictionblock using one or more motion vectors. Video encoder 200 may generallyperform a motion search to identify a reference block that closelymatches the CU, e.g., in terms of differences between the CU and thereference block. Video encoder 200 may calculate a difference metricusing a sum of absolute difference (SAD), sum of squared differences(SSD), mean absolute difference (MAD), mean squared differences (MSD),or other such difference calculations to determine whether a referenceblock closely matches the current CU. In some examples, video encoder200 may predict the current CU using uni-directional prediction orbi-directional prediction.

Some examples of VVC also provide an affine motion compensation mode,which may be considered an inter-prediction mode. In affine motioncompensation mode, video encoder 200 may determine two or more motionvectors that represent non-translational motion, such as zoom in or out,rotation, perspective motion, or other irregular motion types.

To perform intra-prediction, video encoder 200 may select anintra-prediction mode to generate the prediction block. Some examples ofVVC provide sixty-seven intra-prediction modes, including variousdirectional modes, as well as planar mode and DC mode. In general, videoencoder 200 selects an intra-prediction mode that describes neighboringsamples to a current block (e.g., a block of a CU) from which to predictsamples of the current block. Such samples may generally be above, aboveand to the left, or to the left of the current block in the same pictureas the current block, assuming video encoder 200 codes CTUs and CUs inraster scan order (left to right, top to bottom).

Video encoder 200 encodes data representing the prediction mode for acurrent block. For example, for inter-prediction modes, video encoder200 may encode data representing which of the various availableinter-prediction modes is used, as well as motion information for thecorresponding mode. For uni-directional or bi-directionalinter-prediction, for example, video encoder 200 may encode motionvectors using advanced motion vector prediction (AMVP) or merge mode.Video encoder 200 may use similar modes to encode motion vectors foraffine motion compensation mode.

Following prediction, such as intra-prediction or inter-prediction of ablock, video encoder 200 may calculate residual data for the block. Theresidual data, such as a residual block, represents sample by sampledifferences between the block and a prediction block for the block,formed using the corresponding prediction mode. Video encoder 200 mayapply one or more transforms to the residual block, to producetransformed data in a transform domain instead of the sample domain. Forexample, video encoder 200 may apply a discrete cosine transform (DCT),an integer transform, a wavelet transform, or a conceptually similartransform to residual video data. Additionally, video encoder 200 mayapply a secondary transform following the first transform, such as amode-dependent non-separable secondary transform (MDNSST), a signaldependent transform, a Karhunen-Loeve transform (KLT), or the like.Video encoder 200 produces transform coefficients following applicationof the one or more transforms.

As noted above, following any transforms to produce transformcoefficients, video encoder 200 may perform quantization of thetransform coefficients. Quantization generally refers to a process inwhich transform coefficients are quantized to possibly reduce the amountof data used to represent the transform coefficients, providing furthercompression. By performing the quantization process, video encoder 200may reduce the bit depth associated with some or all of the transformcoefficients. For example, video encoder 200 may round an n-bit valuedown to an m-bit value during quantization, where n is greater than m.In some examples, to perform quantization, video encoder 200 may performa bitwise right-shift of the value to be quantized.

Following quantization, video encoder 200 may scan the transformcoefficients, producing a one-dimensional vector from thetwo-dimensional matrix including the quantized transform coefficients.The scan may be designed to place higher energy (and therefore lowerfrequency) transform coefficients at the front of the vector and toplace lower energy (and therefore higher frequency) transformcoefficients at the back of the vector. In some examples, video encoder200 may utilize a predefined scan order to scan the quantized transformcoefficients to produce a serialized vector, and then entropy encode thequantized transform coefficients of the vector. In other examples, videoencoder 200 may perform an adaptive scan. After scanning the quantizedtransform coefficients to form the one-dimensional vector, video encoder200 may entropy encode the one-dimensional vector, e.g., according tocontext-adaptive binary arithmetic coding (CABAC). Video encoder 200 mayalso entropy encode values for syntax elements describing metadataassociated with the encoded video data for use by video decoder 300 indecoding the video data.

To perform CABAC, video encoder 200 may assign a context within acontext model to a symbol to be transmitted. The context may relate to,for example, whether neighboring values of the symbol are zero-valued ornot. The probability determination may be based on a context assigned tothe symbol.

Video encoder 200 may further generate syntax data, such as block-basedsyntax data, picture-based syntax data, and sequence-based syntax data,to video decoder 300, e.g., in a picture header, a block header, a sliceheader, or other syntax data, such as a sequence parameter set (SPS),picture parameter set (PPS), or video parameter set (VPS). Video decoder300 may likewise decode such syntax data to determine how to decodecorresponding video data.

In this manner, video encoder 200 may generate a bitstream includingencoded video data, e.g., syntax elements describing partitioning of apicture into blocks (e.g., CUs) and prediction and/or residualinformation for the blocks. Ultimately, video decoder 300 may receivethe bitstream and decode the encoded video data.

In general, video decoder 300 performs a reciprocal process to thatperformed by video encoder 200 to decode the encoded video data of thebitstream. For example, video decoder 300 may decode values for syntaxelements of the bitstream using CABAC in a manner substantially similarto, albeit reciprocal to, the CABAC encoding process of video encoder200. The syntax elements may define partitioning information forpartitioning of a picture into CTUs, and partitioning of each CTUaccording to a corresponding partition structure, such as a QTBTstructure, to define CUs of the CTU. The syntax elements may furtherdefine prediction and residual information for blocks (e.g., CUs) ofvideo data.

The residual information may be represented by, for example, quantizedtransform coefficients. Video decoder 300 may inverse quantize andinverse transform the quantized transform coefficients of a block toreproduce a residual block for the block. Video decoder 300 uses asignaled prediction mode (intra- or inter-prediction) and relatedprediction information (e.g., motion information for inter-prediction)to form a prediction block for the block. Video decoder 300 may thencombine the prediction block and the residual block (on asample-by-sample basis) to reproduce the original block. Video decoder300 may perform additional processing, such as performing a deblockingprocess to reduce visual artifacts along boundaries of the block.

This disclosure may generally refer to “signaling” certain information,such as syntax elements. The term “signaling” may generally refer to thecommunication of values for syntax elements and/or other data used todecode encoded video data. That is, video encoder 200 may signal valuesfor syntax elements in the bitstream. In general, signaling refers togenerating a value in the bitstream. As noted above, source device 102may transport the bitstream to destination device 116 substantially inreal time, or not in real time, such as might occur when storing syntaxelements to storage device 112 for later retrieval by destination device116.

FIGS. 2A and 2B are conceptual diagrams illustrating an example quadtreebinary tree (QTBT) structure 130, and a corresponding coding tree unit(CTU) 132. The solid lines represent quadtree splitting, and dottedlines indicate binary tree splitting. In each split (i.e., non-leaf)node of the binary tree, one flag is signaled to indicate whichsplitting type (i.e., horizontal or vertical) is used, where 0 indicateshorizontal splitting and 1 indicates vertical splitting in this example.For the quadtree splitting, there is no need to indicate the splittingtype, because quadtree nodes split a block horizontally and verticallyinto 4 sub-blocks with equal size. Accordingly, video encoder 200 mayencode, and video decoder 300 may decode, syntax elements (such assplitting information) for a region tree level of QTBT structure 130(i.e., the solid lines) and syntax elements (such as splittinginformation) for a prediction tree level of QTBT structure 130 (i.e.,the dashed lines). Video encoder 200 may encode, and video decoder 300may decode, video data, such as prediction and transform data, for CUsrepresented by terminal leaf nodes of QTBT structure 130.

In general, CTU 132 of FIG. 2B may be associated with parametersdefining sizes of blocks corresponding to nodes of QTBT structure 130 atthe first and second levels. These parameters may include a CTU size(representing a size of CTU 132 in samples), a minimum quadtree size(MinQTSize, representing a minimum allowed quadtree leaf node size), amaximum binary tree size (MaxBTSize, representing a maximum allowedbinary tree root node size), a maximum binary tree depth (MaxBTDepth,representing a maximum allowed binary tree depth), and a minimum binarytree size (MinBTSize, representing the minimum allowed binary tree leafnode size).

The root node of a QTBT structure corresponding to a CTU may have fourchild nodes at the first level of the QTBT structure, each of which maybe partitioned according to quadtree partitioning. That is, nodes of thefirst level are either leaf nodes (having no child nodes) or have fourchild nodes. The example of QTBT structure 130 represents such nodes asincluding the parent node and child nodes having solid lines forbranches. If nodes of the first level are not larger than the maximumallowed binary tree root node size (MaxBTSize), then the nodes can befurther partitioned by respective binary trees. The binary treesplitting of one node can be iterated until the nodes resulting from thesplit reach the minimum allowed binary tree leaf node size (MinBTSize)or the maximum allowed binary tree depth (MaxBTDepth). The example ofQTBT structure 130 represents such nodes as having dashed lines forbranches. The binary tree leaf node is referred to as a coding unit(CU), which is used for prediction (e.g., intra-picture or inter-pictureprediction) and transform, without any further partitioning. Asdiscussed above, CUs may also be referred to as “video blocks” or“blocks.”

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 (luma samples and two corresponding 64×64 chroma samples),the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, theMinBTSize (for both width and height) is set as 4, and the MaxBTDepth isset as 4. The quadtree partitioning is applied to the CTU first togenerate quad-tree leaf nodes. The quadtree leaf nodes may have a sizefrom 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If thequadtree leaf node is 128×128, the leaf quadtree node will not befurther split by the binary tree, because the size exceeds the MaxBTSize(i.e., 64×64, in this example). Otherwise, the quadtree leaf node willbe further partitioned by the binary tree. Therefore, the quadtree leafnode is also the root node for the binary tree and has the binary treedepth as 0. When the binary tree depth reaches MaxBTDepth (4, in thisexample), no further splitting is permitted. A binary tree node having awidth equal to MinBTSize (4, in this example) implies that no furthervertical splitting (that is, dividing of the width) is permitted forthat binary tree node. Similarly, a binary tree node having a heightequal to MinBTSize implies no further horizontal splitting (that is,dividing of the height) is permitted for that binary tree node. As notedabove, leaf nodes of the binary tree are referred to as CUs, and arefurther processed according to prediction and transform without furtherpartitioning.

In example video coding standards prior to HEVC, only a fixed separabletransform is used where DCT-2 is used both vertically and horizontally.In HEVC, in addition to DCT-2, DST-7 is also employed for 4×4 blocks asa fixed separable transform.

U.S. Patent Publication 2016-0219290-A1, U.S. Patent Publication2018-0020218-A1, and U.S. Patent Publication 2019-0373261-A1 describetechniques for multiple transform selection (MTS). MTS may also bereferred to as Adaptive Multiple Transforms (AMT). An example of MTS in2019-0373261-A1 has been adopted in Joint Video Experts Team (JVET) ofITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, JEM Software,https://jvet.hhi.fraunhofer.de/svn/svn_HMJEMSoftware/tags/HM-16.6-JEM-7.0(JEM 7.0), and later a simplified version of MTS is adopted in VVC Draft10.

FIG. 3 is an illustration of an example LFNST performed by a videoencoder and video decoder (e.g., video encoder 200 and video decoder300), where the use of an LFNST introduces a new stage between separabletransformation and quantization in a codec. As shown in FIG. 3 , at theencoder side (e.g., video encoder 200), transform processing unit 206may first apply a separable transform 500 on a transform block to obtaintransform coefficients. Transform processing unit 206 may then apply anLFNST 502 to a portion (e.g., an LFNST region) of the transformcoefficients of the transform block. As described above, transformprocessing unit 206 may apply a zero-out process in conjunction with theLFNST. Quantization unit 208 may then quantize the resulting transformcoefficients before entropy encoding. Transform processing unit 206 andquantization unit 208 are described in more detail with respect to FIG.12 .

At the decoder side (e.g., video decoder 300), inverse quantization unit306 first inverse quantizes entropy decoded transform coefficients in atransform block. Then inverse transform processing unit 308 of videodecoder 300 applies an inverse LFNST 504 to an LFSNT region of thetransform block. Then inverse transform processing unit 308 applies aninverse separable transform 506 to results of the inverse LFNST toproduce a residual block. Inverse quantization unit 306 and inversetransform processing unit 308 are described in more detail with respectto FIG. 13 .

LFNST, as illustrated in FIG. 3 , is used in JEM-7.0 to further improvethe coding efficiency of MTS, where an implementation of LFNST is basedon HyGT as described in U.S. Patent Publication 2017-0238013-A1. U.S.Patent Publication 2017-0094313-A1, U.S. Patent Publication2017-0238014-A1, U.S. Provisional Patent Application 62/668,105, filed &May 2018, and U.S. Patent Publication 2019-0297351-A1 also describeadditional designs and further details. LFNST has been adopted in VVCDraft 10 and is described in JVET-N0193, Reduced Secondary Transform(RST) (CE6-3.1), 14th Meeting: Geneva, CH, 19-27 Mar. 2019. LFNST mayalso be referred to as non-separable secondary transform (NSST) orsecondary transform.

FIG. 3 shows an illustration of LFNST transform, which may be performedby a video encoder and video decoder where LFNST introduces a new stagebetween separable transformation and quantization in a codec.

The decoding process for LFNST will now be described. The inversetransformation with LFNST involves following main steps as illustratedin FIG. 4 :

-   -   1) The decoded transform coefficients (see subblock 315 in FIG.        4 ) are used as input to the inverse LFNST by first converting        the 2-D block into a 1-D list (or vector) of coefficients via        pre-defined scanning/ordering;    -   2) An inverse LFNST is applied to the 1-D list of input        coefficients and the output coefficients are reorganized into        2-D block via pre-defined scanning/ordering (see subblock 317 in        FIG. 4 );    -   3) The inverse transformed LFNST coefficients are used as input        the separable inverse DCT-2 to obtain reconstructed residuals.

FIG. 4 shows an illustration of inverse transform process when LFNST isused.

In VVC Draft 10, LFNST can be applied to 4×4 and 8×8 subblocks. In bothcases, 16 decoded coefficients in a 4×4 subblock (some of which may benormatively zeroed-out) are input to an inverse LFNST:

-   -   For the 4×4 case, a 16×16 inverse LFNST is used to construct 16        intermediate coefficients before the separable inverse DCT-2 as        shown in FIG. 5 .    -   For the 8×8 case, a 16×48 inverse LFNST is used to construct 48        intermediate coefficients before the separable inverse DCT-2 as        shown in FIG. 6 . Note that 48 intermediate coefficients are        reorganized in an L-shaped pattern.    -   An inverse LFNST processes can be fully defined based on (i) a        transform (i.e., LFNST) matrix and (ii) a reorganization        pattern/scan for intermediate coefficients.    -   The details of the zero-out process in VVC Draft 10 are        described in U.S. patent application Ser. No. 15/931,271, filed        13 May 2020.

FIG. 5 shows an illustration of 4×4 inverse LFNST used to reconstruct 16intermediate coefficients form a list of 16 input coefficients.

For a 4×4 LFNST, the following two patterns/scans are used depending onintra mode:

const  in  g_lfnstRGScan4x4  [16] = {  //  0  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15   0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15};const  int  g_lfnstRGTranScan4x4[16] = {  //  0  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15   0, 1, 2, 3, 4, 5, 6, 7, 8, 8, 9, 10, 11, 12, 13, 14, 15};

where above two patterns/scans indicate the reordering of intermediatecoefficients. For example, g_lfnstRGScan4×4 does not change therow-major reordering of coefficients. However, lfnstRGTranScan4×4reorders by transposing the order of coefficients (e.g., coefficients at1, 2, 3, 6, 7 and 11 are swapped with coefficients at 4, 8, 12, 9, 13and 14, respectively).

For 4×4 LFNST, there are eight 16×16 matrices in VVC Draft 10. These arelisted in Section 8.7.4.3 of 7VET-02001.

FIG. 6 shows an illustration of 8×8 inverse LFNST used to reconstruct 48intermediate coefficients form a list of 16 input coefficients. Theintermediate coefficients are reorganized in an L-shaped pattern.

For an 8×8 LFNST, the following two patterns/scans are used depending onintra mode:

const  int  g_lfnstRGScan8x8  [48] = {  //  0  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  2324  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  45  4647   0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 56, 57, 58, 59};const  int  g_lfnstRGTranScan8x8[48] = {  //  0  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  2324  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46470, 8, 16, 24, 32, 40, 48, 56, 1, 9, 17, 25, 33, 41, 49, 57, 2, 10, 18, 26, 34, 42, 50, 58, 3, 11, 19, 27, 35, 43, 51, 59, 4, 12, 20, 28, 5, 13, 21, 29, 6, 14, 22, 30, 7, 15, 23, 31};where above two patterns/scans indicate the reordering of intermediatecoefficients. Specifically, g_lfnstRGScan8×8 reorganizes 48 intermediatecoefficients in the L-shaped pattern (e.g., the 48th coefficient ismapped to location 59 in FIG. 6 ). The scan lfnstRGTranScan4×4 reordersthe L-shaped pattern by transposing coefficients (e.g., the 48thcoefficient is mapped to location 31 in FIG. 6 ).

For 8×8 LFNST, there are eight 16×48 matrices in VVC Draft 10. These arelisted in Section 8.7.4.3 of JVET-02001.

For both 4×4 and 8×8 LFNST in VVC Draft 10, 8 LFNST matrices are dividedinto 4 sets of LFNST matrices, each of which consist of 2 LFNSTcandidates. Transform matrix used for decoding is determined bysignaling an LFNST index, which identifies the transform matrix usedamong 2 LFNST candidates in a given transform set. In VVC Draft 10, thetransform set is derived based on intra modes. Specifically, it isderived according to the following table

Mapping Rules for Four LFNST Sets in VVC Draft 10

predModeIntra lfnstTrSetIdx predModeIntra < 0 1 0 <= predModeIntra <= 10 2 <= predModeIntra <= 12 1 13 <= predModeIntra <= 23 2 24 <=predModeIntra <= 44 3 45 <= predModeIntra <= 55 2 56 <= predModeIntra <=80 1

This disclosure describes techniques for extending the LFNST designs ofVVC Draft 10. The proposed techniques aim to improve the LFNST design byintroducing additional transform sets and candidates and providetechniques to handle worst-case complexity in terms of number ofmultiplications (i.e., multiplications per coefficient).

This disclosure also describes aspects of an LFNST decoding process.After a list of 2-D dequantized coefficients are obtained (based on acoefficient decoding step), a decoder (e.g., video decoder 300) mayapply an inverse low-frequency non-separable transform (LFNST) toreconstruct a subset of coefficients. Then, an inverse separabletransform (e.g., DCT-2) is used to reconstruct residuals in a 2-Dblock/array (see e.g., FIG. 4 ).

An LFNST decoding process can be specified based on the following:

-   -   Reorganization patterns/scans, which are used to define how        LFNST input and output coefficients are organized.    -   A transform matrix, defining the non-separable transform used        for a subset of decoded coefficients.

In LFNST, a 2-D block/array of decoded coefficients may be organized inorder to align the entries of matrices and the input. As illustrated inFIG. 7 , this can be achieved by:

-   -   i) constructing a 1-D list of coefficients M from the 2-D array        of coefficients, then    -   ii) an inverse LFNST (of size M×N) is applied on the 1-D list to        reconstruct N LFNST coefficients, and    -   iii) the output coefficients are reorganized in a 2-D        block/array, which are input to the separable transform (inverse        DCT-2) reconstructing the residual block.

In VVC Draft 10, the LFNST design allows at most M=16 non-zerocoefficients (located at top-left 4×4 subblock) while the rest of thecoefficients are normatively set to be zeros as shown in FIG. 7 .However, the ideas in this disclosure may be applied for designs thatallow all coefficients to be non-zeros. One example may be the casewhere M=N. In another example, none of the coefficients are normativelyzeroed-out.

FIG. 7 shows an illustration of inverse LFNST process.

Input Reorganization:

-   -   For example, the reorganization can be done in raster-scan (also        called row-major ordering as shown in FIG. 8 ).    -   As another example, the reorganization can be done in        column-major ordering.    -   The reorganization can also depend on block size and intra mode.    -   If a codec (e.g., video encoder 200 or video decoder)        normatively sets a subset of input coefficients to be        zeroed-out, then input coefficient list for LFNST may not        include those zeroed-out coefficients. The reorganization step        only allows coefficients that can be non-zero (i.e.,        coefficients that are not normatively zeroed out).

FIG. 8 shows an example of input reorganization based on raster-scanwith 16 decoded coefficients. FIG. 9 shows an example of inputreorganization based on raster-scan with 64 decoded coefficients.

Transform Matrix:

-   -   The transform (LFNST) matrix can be of size M×N, where        -   M denotes the number of basis vectors and also denotes the            number of rows        -   N denotes the number of reconstructed LFNST coefficients            after applying the transform (also known as the number of            support samples for the transform).    -   The transform matrix entries can be in 8-bit, 9-bit or 10-bit        precision.    -   The signs of all entries in a row of the transform matrix can be        flipped. In other words, a row-vector in the transform matrix        can be multiplied by −1.        -   All rows in the transform matrix can be multiplied by −1.        -   A subset of rows in the transform matrix can be multiplied            by 1 while the other entries are unchanged.            Output Reorganization Patterns/Scans:    -   A 1-D list of N output LFNST coefficients can be reorganized        based on an array (defining a pattern/scan), where each value in        the array corresponds to a position/location in a 2-D block.    -   The values in the array (used for reorganization) can denote        indices of a 2-D block in any pre-defined order.        -   In one example, the index values can correspond to a            position in a 2-D block. Given an index value v, the            corresponding position in the 2-D block can be calculated            as:            r=floor(v/w)  row index:            c=mod(v,w)  column index:        -   where w denotes the width of the LFNST subblock (in VVC            Draft 10, w can be 4 or 8). Based on this formula, the            following array for reorganization pattern corresponds to            the raster-order for 4×4 blocks:

const  int  raster_order  [16] = {  //  0  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15   0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,};

-   -   -   -   so that i-th element in the 1-D coefficient list is                mapped to the row and column positions in 2-D block as                for i=0, 1, . . . 15:                r[i]=floor(raster_order[i]/w)                c[i]=mod(raster_order[i],w)

In the examples above, N is the same value as in M×N and is also equalto the number of reconstructed coefficients at the decoder side. M isthe number of input coefficients associated with M rows of an M×Nmatrix.

N: number of output/reconstructed coefficients that is equal to numberof columns (i.e., N) in the matrix.

This disclosure also describes techniques for extending the 8×8 LFNSTsupport with/without zero-out. The 8×8 LFNST design in VVC Draft 10 canbe extended to have a larger support for reconstructed LFNSTcoefficients.

In one example, the number of support samples can be extended from 48 to64 by keeping the number of allowed coefficients as M=16 as shown inFIG. 10 . Note that in this design both video decoder 300 and videoencoder 200 may be configured such that only up to 16 coefficients canbe non-zero as show in FIG. 8 where input block has 16 samples.

FIG. 9 shows an illustration of 8×8 inverse LFNST used to reconstruct 64intermediate coefficients from 16 input coefficients. The intermediatecoefficients are reorganized in an 8×8-square pattern.

In another example, the number of support samples can be extended from48 to 64 by further extending the number of allowed coefficients to M=64as shown in FIG. 11 . Note that this design does not allow any normativezeroing out of the coefficients (i.e., both video encoder 200 and videodecoder 300 may be configured such that there can be a non-zerocoefficient in any position of the square block).

FIG. 11 shows an example illustration of 8×8 inverse LFNST used toreconstruct 64 intermediate coefficients from 64 input coefficients. Theintermediate coefficients are reorganized in an 8×8-square pattern.

The input reorganization can be done in raster-scan as in FIG. 8 forM=16 or 64 decoded coefficients, and the output reorganizationpatterns/scans are specified in arrays g_lfnstRGScan8×8 andg_lfnstRGTranScan8×8, where one of these arrays is used depending onintra mode. In one example, lfnstRGScan is used for intra modes indexedas 0, 1, 2, 3 . . . 34 and lfnstRGTranScan is used for 35, 36, . . . .

const  int  g_lnfstRGScan8x8  [64] = {   0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,   23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43,   44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63};const  int  g_lfnstRGTranScan8x8[64] = {   0, 8, 16, 24, 32, 40, 48, 56, 1, 9, 17, 25, 33, 41, 49, 57, 2, 10, 18, 26, 34, 42,   50, 58, 3, 11, 19, 27, 35, 43, 51, 59, 4, 12, 20, 28, 36, 44, 52, 60, 5, 13, 21, 29,   37, 45, 53, 61, 6, 14, 22, 30, 38, 46, 54, 62, 7, 15, 23, 31, 39, 47, 55, 63};where above scans correspond to raster scan and transposed order,specifically.

This disclosure also describes techniques for extending the number ofLFNST candidates.

The number of LFNST sets (S) and candidates (C) can be extended to S=35and C=3, and the LFNST set (lfnstTrSetIdx) for a given intra mode can bederived according to the following mapping table.

Mapping Rules for Four LFNST Sets an Extended LFNST Design

predModeIntra lfnstTrSetIdx predModeIntra < 0 2 0 0 1 1 2 2 3 3 4 4 5 56 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 1919 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 28 28 29 29 30 30 3131 32 32 33 33 34 34 35 33 36 32 37 31 38 30 39 29 40 28 41 27 42 26 4325 44 24 45 23 46 22 47 21 48 20 49 19 50 18 51 17 52 16 53 15 54 14 5513 56 12 57 11 58 10 59 9 60 8 61 7 62 6 63 5 64 4 65 3 66 2

For S=35 and C=3, the collection of 105 LFNST matrices g_lfnst4×4 andg_lfnst8×8 can be a wide variety of different matrices.

This disclosure also describes techniques for improving worst-casematrix multiplication handling.

For hardware implementations, a worst-case number of multiplication(per-transform coefficient) is an important complexity criterion. Oneexample technique to reduce worst-case number of multiplications is tonormatively zero-out some of the transform coefficients. However, thezeroing-out process is undesirable unless it is required to meet acertain worst-case number of multiplications.

In VVC Draft 10, the following set of rules are used to maintain theworst-case complexity at 8 multiplications:

-   -   If LFNST is used and TU size is 4×4 or 8×8, then at most 8        non-zero coefficients are allowed, and the rest of the        coefficients are normatively zeroed-out.    -   Otherwise, if LFNST is used, then at most 16 non-zero        coefficients are allowed, and the rest of the coefficients are        normatively zeroed-out.

In the specification text, this is reflected as:nonZeroSize=((nTbW==4&& nTbH==4)∥(nTbW==8&& nTbH==8))?8:16

Let WCM denotes the desired worst-case number of multiplications. Thegeneral formula for determining the number of non-zero coefficientsallowed (i.e., the level of zeroing out) is:

NZ=floor((WCM×nTbW×nTbH)/(N×N)) where nTbH and nTbW denotes the heightand width of a TU, and Nis the transform dimension as described above.This general formula can be reflected in a specification text asfollows:nonZeroSize=(nTbW==nTbH)?NZ:(2×NZ),

where when the TU is square number of allowed coefficients is equal toNZ, otherwise it is equal to 2 times NZ.

If block width and height are equal, then the number of allowed non-zerocoefficients may be equal to NZ. Otherwise, the number of allowednon-zero coefficients may be equal to 2 times NZ (e.g., 2×NZ).

To implement the above described techniques, video encoder 200 and videodecoder 300 may be configured to determine a number of allowed non-zerocoefficients for a block of video data based on a size of the block andobtain a set of dequantized coefficients for the block of video data,wherein the set of dequantized coefficients comprises a first subset ofdequantized coefficients that includes non-zero dequantized coefficientsand a second subset of dequantized coefficients that includes all zerocoefficients. A number of coefficients in the first subset ofdequantized coefficients may be equal to the number of allowed non-zerocoefficients for the block of video data. Video encoder 200 and videodecoder 300 may then apply an inverse LFNST to the first subset ofdequantized coefficients to determine a first intermediate subset ofcoefficients and apply an inverse separable transform to the firstintermediate subset of coefficients and at least a portion of the secondsubset of coefficients to determine a block of reconstructed residualvalues.

FIG. 12 is a block diagram illustrating an example video encoder 200that may perform the techniques of this disclosure. FIG. 12 is providedfor purposes of explanation and should not be considered limiting of thetechniques as broadly exemplified and described in this disclosure. Forpurposes of explanation, this disclosure describes video encoder 200according to the techniques of VVC (ITU-T H.266, under development), andHEVC (ITU-T H.265). However, the techniques of this disclosure may beperformed by video encoding devices that are configured to other videocoding standards.

In the example of FIG. 12 , video encoder 200 includes video data memory230, mode selection unit 202, residual generation unit 204, transformprocessing unit 206, quantization unit 208, inverse quantization unit210, inverse transform processing unit 212, reconstruction unit 214,filter unit 216, decoded picture buffer (DPB) 218, and entropy encodingunit 220. Any or all of video data memory 230, mode selection unit 202,residual generation unit 204, transform processing unit 206,quantization unit 208, inverse quantization unit 210, inverse transformprocessing unit 212, reconstruction unit 214, filter unit 216, DPB 218,and entropy encoding unit 220 may be implemented in one or moreprocessors or in processing circuitry. For instance, the units of videoencoder 200 may be implemented as one or more circuits or logic elementsas part of hardware circuitry, or as part of a processor, ASIC, or FPGA.Moreover, video encoder 200 may include additional or alternativeprocessors or processing circuitry to perform these and other functions.

Video data memory 230 may store video data to be encoded by thecomponents of video encoder 200. Video encoder 200 may receive the videodata stored in video data memory 230 from, for example, video source 104(FIG. 1 ). DPB 218 may act as a reference picture memory that storesreference video data for use in prediction of subsequent video data byvideo encoder 200. Video data memory 230 and DPB 218 may be formed byany of a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. Video datamemory 230 and DPB 218 may be provided by the same memory device orseparate memory devices. In various examples, video data memory 230 maybe on-chip with other components of video encoder 200, as illustrated,or off-chip relative to those components.

In this disclosure, reference to video data memory 230 should not beinterpreted as being limited to memory internal to video encoder 200,unless specifically described as such, or memory external to videoencoder 200, unless specifically described as such. Rather, reference tovideo data memory 230 should be understood as reference memory thatstores video data that video encoder 200 receives for encoding (e.g.,video data for a current block that is to be encoded). Memory 106 ofFIG. 1 may also provide temporary storage of outputs from the variousunits of video encoder 200.

The various units of FIG. 12 are illustrated to assist withunderstanding the operations performed by video encoder 200. The unitsmay be implemented as fixed-function circuits, programmable circuits, ora combination thereof. Fixed-function circuits refer to circuits thatprovide particular functionality, and are preset on the operations thatcan be performed. Programmable circuits refer to circuits that can beprogrammed to perform various tasks, and provide flexible functionalityin the operations that can be performed. For instance, programmablecircuits may execute software or firmware that cause the programmablecircuits to operate in the manner defined by instructions of thesoftware or firmware. Fixed-function circuits may execute softwareinstructions (e.g., to receive parameters or output parameters), but thetypes of operations that the fixed-function circuits perform aregenerally immutable. In some examples, one or more of the units may bedistinct circuit blocks (fixed-function or programmable), and in someexamples, one or more of the units may be integrated circuits.

Video encoder 200 may include arithmetic logic units (ALUs), elementaryfunction units (EFUs), digital circuits, analog circuits, and/orprogrammable cores, formed from programmable circuits. In examples wherethe operations of video encoder 200 are performed using softwareexecuted by the programmable circuits, memory 106 (FIG. 1 ) may storethe instructions (e.g., object code) of the software that video encoder200 receives and executes, or another memory within video encoder 200(not shown) may store such instructions.

Video data memory 230 is configured to store received video data. Videoencoder 200 may retrieve a picture of the video data from video datamemory 230 and provide the video data to residual generation unit 204and mode selection unit 202. Video data in video data memory 230 may beraw video data that is to be encoded.

Mode selection unit 202 includes a motion estimation unit 222, a motioncompensation unit 224, and an intra-prediction unit 226. Mode selectionunit 202 may include additional functional units to perform videoprediction in accordance with other prediction modes. As examples, modeselection unit 202 may include a palette unit, an intra-block copy unit(which may be part of motion estimation unit 222 and/or motioncompensation unit 224), an affine unit, a linear model (LM) unit, or thelike.

Mode selection unit 202 generally coordinates multiple encoding passesto test combinations of encoding parameters and resultingrate-distortion values for such combinations. The encoding parametersmay include partitioning of CTUs into CUs, prediction modes for the CUs,transform types for residual data of the CUs, quantization parametersfor residual data of the CUs, and so on. Mode selection unit 202 mayultimately select the combination of encoding parameters havingrate-distortion values that are better than the other testedcombinations.

Video encoder 200 may partition a picture retrieved from video datamemory 230 into a series of CTUs, and encapsulate one or more CTUswithin a slice. Mode selection unit 202 may partition a CTU of thepicture in accordance with a tree structure, such as the QTBT structureor the quad-tree structure of HEVC described above. As described above,video encoder 200 may form one or more CUs from partitioning a CTUaccording to the tree structure. Such a CU may also be referred togenerally as a “video block” or “block.”

In general, mode selection unit 202 also controls the components thereof(e.g., motion estimation unit 222, motion compensation unit 224, andintra-prediction unit 226) to generate a prediction block for a currentblock (e.g., a current CU, or in HEVC, the overlapping portion of a PUand a TU). For inter-prediction of a current block, motion estimationunit 222 may perform a motion search to identify one or more closelymatching reference blocks in one or more reference pictures (e.g., oneor more previously coded pictures stored in DPB 218). In particular,motion estimation unit 222 may calculate a value representative of howsimilar a potential reference block is to the current block, e.g.,according to sum of absolute difference (SAD), sum of squareddifferences (SSD), mean absolute difference (MAD), mean squareddifferences (MSD), or the like. Motion estimation unit 222 may generallyperform these calculations using sample-by-sample differences betweenthe current block and the reference block being considered. Motionestimation unit 222 may identify a reference block having a lowest valueresulting from these calculations, indicating a reference block thatmost closely matches the current block.

Motion estimation unit 222 may form one or more motion vectors (MVs)that defines the positions of the reference blocks in the referencepictures relative to the position of the current block in a currentpicture. Motion estimation unit 222 may then provide the motion vectorsto motion compensation unit 224. For example, for uni-directionalinter-prediction, motion estimation unit 222 may provide a single motionvector, whereas for bi-directional inter-prediction, motion estimationunit 222 may provide two motion vectors. Motion compensation unit 224may then generate a prediction block using the motion vectors. Forexample, motion compensation unit 224 may retrieve data of the referenceblock using the motion vector. As another example, if the motion vectorhas fractional sample precision, motion compensation unit 224 mayinterpolate values for the prediction block according to one or moreinterpolation filters. Moreover, for bi-directional inter-prediction,motion compensation unit 224 may retrieve data for two reference blocksidentified by respective motion vectors and combine the retrieved data,e.g., through sample-by-sample averaging or weighted averaging.

As another example, for intra-prediction, or intra-prediction coding,intra-prediction unit 226 may generate the prediction block from samplesneighboring the current block. For example, for directional modes,intra-prediction unit 226 may generally mathematically combine values ofneighboring samples and populate these calculated values in the defineddirection across the current block to produce the prediction block. Asanother example, for DC mode, intra-prediction unit 226 may calculate anaverage of the neighboring samples to the current block and generate theprediction block to include this resulting average for each sample ofthe prediction block.

Mode selection unit 202 provides the prediction block to residualgeneration unit 204. Residual generation unit 204 receives a raw,unencoded version of the current block from video data memory 230 andthe prediction block from mode selection unit 202. Residual generationunit 204 calculates sample-by-sample differences between the currentblock and the prediction block. The resulting sample-by-sampledifferences define a residual block for the current block. In someexamples, residual generation unit 204 may also determine differencesbetween sample values in the residual block to generate a residual blockusing residual differential pulse code modulation (RDPCM). In someexamples, residual generation unit 204 may be formed using one or moresubtractor circuits that perform binary subtraction.

In examples where mode selection unit 202 partitions CUs into PUs, eachPU may be associated with a luma prediction unit and correspondingchroma prediction units. Video encoder 200 and video decoder 300 maysupport PUs having various sizes. As indicated above, the size of a CUmay refer to the size of the luma coding block of the CU and the size ofa PU may refer to the size of a luma prediction unit of the PU. Assumingthat the size of a particular CU is 2N×2N, video encoder 200 may supportPU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder200 and video decoder 300 may also support asymmetric partitioning forPU sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N for inter prediction.

In examples where mode selection unit 202 does not further partition aCU into PUs, each CU may be associated with a luma coding block andcorresponding chroma coding blocks. As above, the size of a CU may referto the size of the luma coding block of the CU. The video encoder 200and video decoder 300 may support CU sizes of 2N×2N, 2N×N, or N×2N.

For other video coding techniques such as an intra-block copy modecoding, an affine-mode coding, and linear model (LM) mode coding, assome examples, mode selection unit 202, via respective units associatedwith the coding techniques, generates a prediction block for the currentblock being encoded. In some examples, such as palette mode coding, modeselection unit 202 may not generate a prediction block, and insteadgenerate syntax elements that indicate the manner in which toreconstruct the block based on a selected palette. In such modes, modeselection unit 202 may provide these syntax elements to entropy encodingunit 220 to be encoded.

As described above, residual generation unit 204 receives the video datafor the current block and the corresponding prediction block. Residualgeneration unit 204 then generates a residual block for the currentblock. To generate the residual block, residual generation unit 204calculates sample-by-sample differences between the prediction block andthe current block.

Transform processing unit 206 applies one or more transforms to theresidual block to generate a block of transform coefficients (referredto herein as a “transform coefficient block”). Transform processing unit206 may apply various transforms to a residual block to form thetransform coefficient block. For example, transform processing unit 206may apply a discrete cosine transform (DCT), a directional transform, aKarhunen-Loeve transform (KLT), or a conceptually similar transform to aresidual block. In some examples, transform processing unit 206 mayperform multiple transforms to a residual block, e.g., a primarytransform and a secondary transform, such as a rotational transform.Transform processing unit 206 may, for example, apply LFNSTs andseparable transforms in the manner described in this disclosure. Inaccordance with the techniques of this disclosure, transform processingunit 206 may, for example, determine a number of allowed non-zerocoefficients for a block of video data based on a size of the block and,as part of applying LFNSTs and separable transforms, perform a zeroingout process based on the determined number of allowed non-zerocoefficients. In some examples, transform processing unit 206 does notapply transforms to a residual block.

Quantization unit 208 may quantize the transform coefficients in atransform coefficient block, to produce a quantized transformcoefficient block. Quantization unit 208 may quantize transformcoefficients of a transform coefficient block according to aquantization parameter (QP) value associated with the current block.Video encoder 200 (e.g., via mode selection unit 202) may adjust thedegree of quantization applied to the transform coefficient blocksassociated with the current block by adjusting the QP value associatedwith the CU. Quantization may introduce loss of information, and thus,quantized transform coefficients may have lower precision than theoriginal transform coefficients produced by transform processing unit206.

Inverse quantization unit 210 and inverse transform processing unit 212may apply inverse quantization and inverse transforms to a quantizedtransform coefficient block, respectively, to reconstruct a residualblock from the transform coefficient block. Inverse transform processingunit 212 may, for example, apply inverse LFNSTs and inverse separabletransforms in the manner described in this disclosure. In accordancewith the techniques of this disclosure, inverse transform processingunit 212 may, for example, determine a number of allowed non-zerocoefficients for a block of video data based on a size of the block and,as part of applying inverse LFNSTs and inverse separable transforms,perform a zeroing out process based on the determined number of allowednon-zero coefficients.

Reconstruction unit 214 may produce a reconstructed block correspondingto the current block (albeit potentially with some degree of distortion)based on the reconstructed residual block and a prediction blockgenerated by mode selection unit 202. For example, reconstruction unit214 may add samples of the reconstructed residual block to correspondingsamples from the prediction block generated by mode selection unit 202to produce the reconstructed block.

Filter unit 216 may perform one or more filter operations onreconstructed blocks. For example, filter unit 216 may performdeblocking operations to reduce blockiness artifacts along edges of CUs.Operations of filter unit 216 may be skipped, in some examples.

Video encoder 200 stores reconstructed blocks in DPB 218. For instance,in examples where operations of filter unit 216 are not performed,reconstruction unit 214 may store reconstructed blocks to DPB 218. Inexamples where operations of filter unit 216 are performed, filter unit216 may store the filtered reconstructed blocks to DPB 218. Motionestimation unit 222 and motion compensation unit 224 may retrieve areference picture from DPB 218, formed from the reconstructed (andpotentially filtered) blocks, to inter-predict blocks of subsequentlyencoded pictures. In addition, intra-prediction unit 226 may usereconstructed blocks in DPB 218 of a current picture to intra-predictother blocks in the current picture.

In general, entropy encoding unit 220 may entropy encode syntax elementsreceived from other functional components of video encoder 200. Forexample, entropy encoding unit 220 may entropy encode quantizedtransform coefficient blocks from quantization unit 208. As anotherexample, entropy encoding unit 220 may entropy encode prediction syntaxelements (e.g., motion information for inter-prediction or intra-modeinformation for intra-prediction) from mode selection unit 202. Entropyencoding unit 220 may perform one or more entropy encoding operations onthe syntax elements, which are another example of video data, togenerate entropy-encoded data. For example, entropy encoding unit 220may perform a context-adaptive variable length coding (CAVLC) operation,a CABAC operation, a variable-to-variable (V2V) length coding operation,a syntax-based context-adaptive binary arithmetic coding (SBAC)operation, a Probability Interval Partitioning Entropy (PIPE) codingoperation, an Exponential-Golomb encoding operation, or another type ofentropy encoding operation on the data. In some examples, entropyencoding unit 220 may operate in bypass mode where syntax elements arenot entropy encoded.

Video encoder 200 may output a bitstream that includes the entropyencoded syntax elements needed to reconstruct blocks of a slice orpicture. In particular, entropy encoding unit 220 may output thebitstream.

The operations described above are described with respect to a block.Such description should be understood as being operations for a lumacoding block and/or chroma coding blocks. As described above, in someexamples, the luma coding block and chroma coding blocks are luma andchroma components of a CU. In some examples, the luma coding block andthe chroma coding blocks are luma and chroma components of a PU.

In some examples, operations performed with respect to a luma codingblock need not be repeated for the chroma coding blocks. As one example,operations to identify a motion vector (MV) and reference picture for aluma coding block need not be repeated for identifying a MV andreference picture for the chroma blocks. Rather, the MV for the lumacoding block may be scaled to determine the MV for the chroma blocks,and the reference picture may be the same. As another example, theintra-prediction process may be the same for the luma coding block andthe chroma coding blocks.

FIG. 13 is a block diagram illustrating an example video decoder 300that may perform the techniques of this disclosure. FIG. 13 is providedfor purposes of explanation and is not limiting on the techniques asbroadly exemplified and described in this disclosure. For purposes ofexplanation, this disclosure describes video decoder 300 according tothe techniques of VVC (ITU-T H.266, under development), and HEVC (ITU-TH.265). However, the techniques of this disclosure may be performed byvideo coding devices that are configured to other video codingstandards.

In the example of FIG. 13 , video decoder 300 includes coded picturebuffer (CPB) memory 320, entropy decoding unit 302, predictionprocessing unit 304, inverse quantization unit 306, inverse transformprocessing unit 308, reconstruction unit 310, filter unit 312, anddecoded picture buffer (DPB) 314. Any or all of CPB memory 320, entropydecoding unit 302, prediction processing unit 304, inverse quantizationunit 306, inverse transform processing unit 308, reconstruction unit310, filter unit 312, and DPB 314 may be implemented in one or moreprocessors or in processing circuitry. For instance, the units of videodecoder 300 may be implemented as one or more circuits or logic elementsas part of hardware circuitry, or as part of a processor, ASIC, or FPGA.Moreover, video decoder 300 may include additional or alternativeprocessors or processing circuitry to perform these and other functions.

Prediction processing unit 304 includes motion compensation unit 316 andintra-prediction unit 318. Prediction processing unit 304 may includeadditional units to perform prediction in accordance with otherprediction modes. As examples, prediction processing unit 304 mayinclude a palette unit, an intra-block copy unit (which may form part ofmotion compensation unit 316), an affine unit, a linear model (LM) unit,or the like. In other examples, video decoder 300 may include more,fewer, or different functional components.

CPB memory 320 may store video data, such as an encoded video bitstream,to be decoded by the components of video decoder 300. The video datastored in CPB memory 320 may be obtained, for example, fromcomputer-readable medium 110 (FIG. 1 ). CPB memory 320 may include a CPBthat stores encoded video data (e.g., syntax elements) from an encodedvideo bitstream. Also, CPB memory 320 may store video data other thansyntax elements of a coded picture, such as temporary data representingoutputs from the various units of video decoder 300. DPB 314 generallystores decoded pictures, which video decoder 300 may output and/or useas reference video data when decoding subsequent data or pictures of theencoded video bitstream. CPB memory 320 and DPB 314 may be formed by anyof a variety of memory devices, such as DRAM, including SDRAM, MRAM,RRAM, or other types of memory devices. CPB memory 320 and DPB 314 maybe provided by the same memory device or separate memory devices. Invarious examples, CPB memory 320 may be on-chip with other components ofvideo decoder 300, or off-chip relative to those components.

Additionally or alternatively, in some examples, video decoder 300 mayretrieve coded video data from memory 120 (FIG. 1 ). That is, memory 120may store data as discussed above with CPB memory 320. Likewise, memory120 may store instructions to be executed by video decoder 300, whensome or all of the functionality of video decoder 300 is implemented insoftware to be executed by processing circuitry of video decoder 300.

The various units shown in FIG. 13 are illustrated to assist withunderstanding the operations performed by video decoder 300. The unitsmay be implemented as fixed-function circuits, programmable circuits, ora combination thereof. Similar to FIG. 12 , fixed-function circuitsrefer to circuits that provide particular functionality, and are preseton the operations that can be performed. Programmable circuits refer tocircuits that can be programmed to perform various tasks, and provideflexible functionality in the operations that can be performed. Forinstance, programmable circuits may execute software or firmware thatcause the programmable circuits to operate in the manner defined byinstructions of the software or firmware. Fixed-function circuits mayexecute software instructions (e.g., to receive parameters or outputparameters), but the types of operations that the fixed-functioncircuits perform are generally immutable. In some examples, one or moreof the units may be distinct circuit blocks (fixed-function orprogrammable), and in some examples, one or more of the units may beintegrated circuits.

Video decoder 300 may include ALUs, EFUs, digital circuits, analogcircuits, and/or programmable cores formed from programmable circuits.In examples where the operations of video decoder 300 are performed bysoftware executing on the programmable circuits, on-chip or off-chipmemory may store instructions (e.g., object code) of the software thatvideo decoder 300 receives and executes.

Entropy decoding unit 302 may receive encoded video data from the CPBand entropy decode the video data to reproduce syntax elements.Prediction processing unit 304, inverse quantization unit 306, inversetransform processing unit 308, reconstruction unit 310, and filter unit312 may generate decoded video data based on the syntax elementsextracted from the bitstream.

In general, video decoder 300 reconstructs a picture on a block-by-blockbasis. Video decoder 300 may perform a reconstruction operation on eachblock individually (where the block currently being reconstructed, i.e.,decoded, may be referred to as a “current block”).

Entropy decoding unit 302 may entropy decode syntax elements definingquantized transform coefficients of a quantized transform coefficientblock, as well as transform information, such as a quantizationparameter (QP) and/or transform mode indication(s). Inverse quantizationunit 306 may use the QP associated with the quantized transformcoefficient block to determine a degree of quantization and, likewise, adegree of inverse quantization for inverse quantization unit 306 toapply. Inverse quantization unit 306 may, for example, perform a bitwiseleft-shift operation to inverse quantize the quantized transformcoefficients. Inverse quantization unit 306 may thereby form a transformcoefficient block including transform coefficients.

After inverse quantization unit 306 forms the transform coefficientblock, inverse transform processing unit 308 may apply one or moreinverse transforms to the transform coefficient block to generate aresidual block associated with the current block. For example, inversetransform processing unit 308 may apply an inverse DCT, an inverseinteger transform, an inverse Karhunen-Loeve transform (KLT), an inverserotational transform, an inverse directional transform, or anotherinverse transform to the transform coefficient block. Inverse transformprocessing unit 308 may apply inverse LFNSTs and inverse separabletransforms in the manner described in this disclosure. In accordancewith the techniques of this disclosure, inverse transform processingunit 308 may, for example, determine a number of allowed non-zerocoefficients for a block of video data based on a size of the block and,as part of applying inverse LFNSTs and inverse separable transforms,perform a zeroing out process based on the determined number of allowednon-zero coefficients.

Furthermore, prediction processing unit 304 generates a prediction blockaccording to prediction information syntax elements that were entropydecoded by entropy decoding unit 302. For example, if the predictioninformation syntax elements indicate that the current block isinter-predicted, motion compensation unit 316 may generate theprediction block. In this case, the prediction information syntaxelements may indicate a reference picture in DPB 314 from which toretrieve a reference block, as well as a motion vector identifying alocation of the reference block in the reference picture relative to thelocation of the current block in the current picture. Motioncompensation unit 316 may generally perform the inter-prediction processin a manner that is substantially similar to that described with respectto motion compensation unit 224 (FIG. 12 ).

As another example, if the prediction information syntax elementsindicate that the current block is intra-predicted, intra-predictionunit 318 may generate the prediction block according to anintra-prediction mode indicated by the prediction information syntaxelements. Again, intra-prediction unit 318 may generally perform theintra-prediction process in a manner that is substantially similar tothat described with respect to intra-prediction unit 226 (FIG. 12 ).Intra-prediction unit 318 may retrieve data of neighboring samples tothe current block from DPB 314.

Reconstruction unit 310 may reconstruct the current block using theprediction block and the residual block. For example, reconstructionunit 310 may add samples of the residual block to corresponding samplesof the prediction block to reconstruct the current block.

Filter unit 312 may perform one or more filter operations onreconstructed blocks. For example, filter unit 312 may performdeblocking operations to reduce blockiness artifacts along edges of thereconstructed blocks. Operations of filter unit 312 are not necessarilyperformed in all examples.

Video decoder 300 may store the reconstructed blocks in DPB 314. Forinstance, in examples where operations of filter unit 312 are notperformed, reconstruction unit 310 may store reconstructed blocks to DPB314. In examples where operations of filter unit 312 are performed,filter unit 312 may store the filtered reconstructed blocks to DPB 314.As discussed above, DPB 314 may provide reference information, such assamples of a current picture for intra-prediction and previously decodedpictures for subsequent motion compensation, to prediction processingunit 304. Moreover, video decoder 300 may output decoded pictures (e.g.,decoded video) from DPB 314 for subsequent presentation on a displaydevice, such as display device 118 of FIG. 1 .

FIG. 14 is a flowchart illustrating an example process for encoding acurrent block in accordance with the techniques of this disclosure. Thecurrent block may comprise a current CU. Although described with respectto video encoder 200 (FIGS. 1 and 12 ), it should be understood thatother devices may be configured to perform a process similar to that ofFIG. 14 .

In this example, video encoder 200 initially predicts the current block(350). For example, video encoder 200 may form a prediction block forthe current block. Video encoder 200 may then calculate a residual blockfor the current block (352). To calculate the residual block, videoencoder 200 may calculate a difference between the original, unencodedblock and the prediction block for the current block. Video encoder 200may then transform the residual block and quantize transformcoefficients of the residual block (354). Next, video encoder 200 mayscan the quantized transform coefficients of the residual block (356).During the scan, or following the scan, video encoder 200 may entropyencode the transform coefficients (358). For example, video encoder 200may encode the transform coefficients using CAVLC or CABAC. Videoencoder 200 may then output the entropy encoded data of the block (360).

FIG. 15 is a flowchart illustrating an example process for decoding acurrent block of video data in accordance with the techniques of thisdisclosure. The current block may comprise a current CU. Althoughdescribed with respect to video decoder 300 (FIGS. 1 and 13 ), it shouldbe understood that other devices may be configured to perform a processsimilar to that of FIG. 15 .

Video decoder 300 may receive entropy encoded data for the currentblock, such as entropy encoded prediction information and entropyencoded data for transform coefficients of a residual blockcorresponding to the current block (370). Video decoder 300 may entropydecode the entropy encoded data to determine prediction information forthe current block and to reproduce transform coefficients of theresidual block (372). Video decoder 300 may predict the current block(374), e.g., using an intra- or inter-prediction mode as indicated bythe prediction information for the current block, to calculate aprediction block for the current block. Video decoder 300 may theninverse scan the reproduced transform coefficients (376), to create ablock of quantized transform coefficients. Video decoder 300 may theninverse quantize the transform coefficients and apply an inversetransform to the transform coefficients to produce a residual block(378). As part of applying the inverse transform, video decoder 300 may,for example, obtain a set of dequantized coefficients for a block ofvideo data, where the set of dequantized coefficients includes a firstsubset of dequantized coefficients that includes non-zero dequantizedcoefficients and a second subset of dequantized coefficients thatincludes all zero coefficients. Video decoder 300 may apply an inverseLFNST to the first subset of dequantized coefficients to determine afirst intermediate subset of coefficients and apply an inverse separabletransform to the first intermediate subset of coefficients and at leasta portion of the second subset of coefficients to determine a block ofreconstructed residual values. Video decoder 300 may ultimately decodethe current block by combining the prediction block and the residualblock (380).

FIG. 16 is a flowchart illustrating an example process for decoding acurrent block of video data in accordance with the techniques of thisdisclosure. The current block may comprise a current CU. Althoughdescribed with respect to video decoder 300 (FIGS. 1 and 13 ), it shouldbe understood that other devices may be configured to perform a processsimilar to that of FIG. 16 . For example, the video decoding loop of avideo encoding process performed by video encoder 200 may also performthe techniques of FIG. 16 .

Video decoder 300 determines a number of allowed non-zero coefficientsfor a block of video data based on a size of the block (400). Videodecoder 300 may determine the number of allowed non-zero coefficientsaccording to the equation NZ=floor((WCM×nTbW×nTbH)/(N×N)), where nTbHrepresents a height of the block of reconstructed values, nTbWrepresents a width of the block of reconstructed values, NZ equals thenumber of allowed non-zero coefficients, and WCM represents a constantvalue. The first subset of dequantized coefficients may, for example,have NZ coefficients. In some examples, M may equal 2×NZ, and N mayequal 64.

Video decoder 300 obtains a set of dequantized coefficients for a blockof video data, wherein the set of dequantized coefficients comprises afirst subset of dequantized coefficients that includes non-zerodequantized coefficients and a second subset of dequantized coefficientsthat includes all zero coefficients (402). A number of coefficients inthe first subset of dequantized coefficients may be equal to the numberof allowed non-zero coefficients for the block of video data.

Video decoder 300 applies an inverse LFNST to the first subset ofdequantized coefficients to determine a first intermediate subset ofcoefficients (404). Video decoder 300 may, for example, determine anintra prediction mode for the block of video data; based on the intraprediction mode, determine a set of inverse LFNST candidates from aplurality of sets; and select the inverse LFNST from the determined setof inverse LFNST candidates. The set of inverse LFNST candidates may,for example, includes 3 candidates and may be determined from aplurality of sets that includes 35 sets. The first subset of dequantizedcoefficients may include M dequantized coefficients, and the firstintermediate subset of coefficients may include N coefficients, with Mand N being integer values. As discussed above, in some example, M mayequal 16, and N equal 64. In other examples, M may equal 64, and N mayequal 64.

Video decoder 300 applies an inverse separable transform to the firstintermediate subset of coefficients and at least a portion of the secondsubset of coefficients to determine a block of reconstructed residualvalues (406). Video decoder 300 may add the block of reconstructedresidual values to a prediction block to form a reconstructed block;apply one or more filters to the reconstructed block to determine afiltered reconstructed block; and output decoded video data thatincludes the filtered reconstructed block.

The following clauses represent implementations of the techniques anddevices described above.

Clause 1. A method of decoding video data, the method comprising:obtaining a set of dequantized coefficients for a block of video data,wherein the set of dequantized coefficients comprises a first subset ofdequantized coefficients that includes non-zero dequantized coefficientsand a second subset of dequantized coefficients that includes all zerocoefficients; applying an inverse low-frequency non-separable transform(LFNST) to the first subset of dequantized coefficients to determine afirst intermediate subset of coefficients; and applying an inverseseparable transform to the first intermediate subset of coefficients andat least a portion of the second subset of coefficients to determine ablock of reconstructed residual values.

Clause 2. The method of clause 1, wherein the first subset ofdequantized coefficients M comprises dequantized coefficients, andwherein first intermediate subset of coefficients comprises Ncoefficients, wherein M and N are integer values.

Clause 3. The method of clause 2, wherein M=16 and N=64.

Clause 4. The method of clause 2, wherein M=64 and N=64.

Clause 5. The method of any of clauses 1-4, further comprising:determining an intra prediction mode for the block of video data; basedon the intra prediction mode, determining a set of inverse LFNSTcandidates from a plurality of sets; and selecting the inverse LFNSTfrom the determined set of inverse LFNST candidates.

Clause 6. The method of clause 5, wherein the set of inverse LFNSTcandidates includes 3 candidates.

Clause 7. The method of clause 5 or 6, wherein the plurality of setsincludes 35 sets.

Clause 8. The method of any of clauses 2-7, further comprising:determining a minimum number of allowed non-zero coefficients accordingto the equation NZ=floor((WCM×nTbW×nTbH)/(N×N)), wherein nTbH representsa height of the block of reconstructed values, nTbW represents a widthof the block of reconstructed values, NZ equals the minimum number ofallowed non-zero coefficients and WCM represents a constant value.

Clause 9. The method of clause 8, wherein the first subset ofdequantized coefficients has NZ coefficients.

Clause 10. The method of clauses 2 and 8 or clauses 2 and 9, whereinM=2×NZ and N=64.

Clause 11. A device for coding video data, the device comprising one ormore means for performing the method of any of clauses 1-10.

Clause 12. The device of clause 11, wherein the one or more meanscomprise one or more processors implemented in circuitry.

Clause 13. The device of any of clauses 11 and 12, further comprising amemory to store the video data.

Clause 14. The device of any of clauses 11-13, further comprising adisplay configured to display decoded video data.

Clause 15. The device of any of clauses 11-14, wherein the devicecomprises one or more of a camera, a computer, a mobile device, abroadcast receiver device, or a set-top box.

Clause 16. The device of any of clauses 11-15, wherein the devicecomprises a video decoder.

Clause 17. The device of any of clauses 11-16, wherein the devicecomprises a video encoder.

Clause 18. A computer-readable storage medium having stored thereoninstructions that, when executed, cause one or more processors toperform the method of any of clauses 1-10.

Clause 19. A method of decoding video data, the method comprising:determining a number of allowed non-zero coefficients for a block ofvideo data based on a size of the block; obtaining a set of dequantizedcoefficients for the block of video data, wherein the set of dequantizedcoefficients comprises a first subset of dequantized coefficients thatincludes non-zero dequantized coefficients and a second subset ofdequantized coefficients that includes all zero coefficients, wherein anumber of coefficients in the first subset of dequantized coefficientsis equal to the number of allowed non-zero coefficients for the block ofvideo data; applying an inverse low-frequency non-separable transform(LFNST) to the first subset of dequantized coefficients to determine afirst intermediate subset of coefficients; and applying an inverseseparable transform to the first intermediate subset of coefficients andat least a portion of the second subset of coefficients to determine ablock of reconstructed residual values.

Clause 20. The method of clause 19, further comprising: determining anintra prediction mode for the block of video data; based on the intraprediction mode, determining a set of inverse LFNST candidates from aplurality of sets; and selecting the inverse LFNST from the determinedset of inverse LFNST candidates.

Clause 21. The method of clause 20, wherein the set of inverse LFNSTcandidates includes 3 candidates.

Clause 22. The method of clause 20 or 21, wherein the plurality of setsincludes 35 sets.

Clause 23. The method of any of clauses 19-22, wherein the first subsetof dequantized coefficients comprises M dequantized coefficients, andwherein first intermediate subset of coefficients comprises Ncoefficients, wherein M and N are integer values.

Clause 24. The method of clause 23, wherein M=16 and N=64.

Clause 25. The method of clause 23, wherein M=64 and N=64.

Clause 26. The method of any of clauses 23-25, further comprising:determining the number of allowed non-zero coefficients according to theequation NZ=floor((WCM×nTbW×nTbH)/(N×N)), wherein nTbH represents aheight of the block of reconstructed values, nTbW represents a width ofthe block of reconstructed values, NZ equals the number of allowednon-zero coefficients and WCM represents a constant value.

Clause 27. The method of clause 26, wherein the first subset ofdequantized coefficients has NZ coefficients.

Clause 28. The method of clause 26 or 27, wherein M=2×NZ and N=64.

Clause 29. The method of any of clauses 19-28, further comprising:adding the block of reconstructed residual values to a prediction blockto form a reconstructed block; applying one or more filters to thereconstructed block to determine a filtered reconstructed block; andoutputting decoded video data that includes the filtered reconstructedblock.

Clause 30. The method of any of clauses 19-29 wherein the method isperformed as part of a video encoding process.

Clause 31. A device for decoding video data, the device comprising: amemory configured to store video data; one or more processorsimplemented in circuitry and configured to: determine a number ofallowed non-zero coefficients for a block of video data based on a sizeof the block; obtain a set of dequantized coefficients for the block ofvideo data, wherein the set of dequantized coefficients comprises afirst subset of dequantized coefficients that includes non-zerodequantized coefficients and a second subset of dequantized coefficientsthat includes all zero coefficients, wherein a number of coefficients inthe first subset of dequantized coefficients is equal to the number ofallowed non-zero coefficients for the block of video data; apply aninverse low-frequency non-separable transform (LFNST) to the firstsubset of dequantized coefficients to determine a first intermediatesubset of coefficients; and apply an inverse separable transform to thefirst intermediate subset of coefficients and at least a portion of thesecond subset of coefficients to determine a block of reconstructedresidual values.

Clause 32. The device of clause 31, wherein the one or more processorsare further configured to: determine an intra prediction mode for theblock of video data; based on the intra prediction mode, determine a setof inverse LFNST candidates from a plurality of sets; and select theinverse LFNST from the determined set of inverse LFNST candidates.

Clause 33. The device of clause 32, wherein the set of inverse LFNSTcandidates includes 3 candidates.

Clause 34. The device of clause 32, wherein the plurality of setsincludes 35 sets.

Clause 35. The device of any of clauses 31-34, wherein the first subsetof dequantized coefficients comprises M dequantized coefficients, andwherein first intermediate subset of coefficients comprises Ncoefficients, wherein M and N are integer values.

Clause 36. The device of clause 35, wherein M=16 and N=64.

Clause 37. The device of clause 35, wherein M=64 and N=64.

Clause 38. The device of any of clauses 35-37, wherein the one or moreprocessors are further configured to: determine the number of allowednon-zero coefficients according to the equationNZ=floor((WCM×nTbW×nTbH)/(N×N)), wherein nTbH represents a height of theblock of reconstructed values, nTbW represents a width of the block ofreconstructed values, NZ equals the number of allowed non-zerocoefficients and WCM represents a constant value.

Clause 39. The device of clause 35, wherein the first subset ofdequantized coefficients has NZ coefficients.

Clause 40. The device of clause 35 or 36, wherein M=2×NZ and N=64.

Clause 41. The device of any of clauses 31-41, wherein the one or moreprocessors are further configured to: add the block of reconstructedresidual values to a prediction block to form a reconstructed block;apply one or more filters to the reconstructed block to determine afiltered reconstructed block; and output decoded video data thatincludes the filtered reconstructed block.

Clause 42. The device of any of clauses 31-41, wherein the devicecomprises a wireless communication device, further comprising a receiverconfigured to receive encoded video data.

Clause 43. The device of clause 42, wherein the wireless communicationdevice comprises a telephone handset and wherein the receiver isconfigured to demodulate, according to a wireless communicationstandard, a signal comprising the encoded video data.

Clause 44. The device of any of clauses 31-43, further comprising: adisplay configured to display decoded video data.

Clause 45. The device of any of clauses 31-44, wherein the devicecomprises one or more of a camera, a computer, a mobile device, abroadcast receiver device, or a set-top box.

Clause 46. The device of any of clauses 31-45, further comprising: acamera configured to capture video data.

Clause 47. The device of any of clauses 31-46, wherein the devicecomprises a wireless communication device, further comprising atransmitter configured to transmit encoded video data.

Clause 48. The device of clause 47, wherein the wireless communicationdevice comprises a telephone handset and wherein the transmitter isconfigured to modulate, according to a wireless communication standard,a signal comprising the encoded video data.

Clause 49. A computer-readable storage medium storing instructions thatwhen executed by one or more processors cause the one or more processorsto: determine a number of allowed non-zero coefficients for a block ofvideo data based on a size of the block; obtain a set of dequantizedcoefficients for the block of video data, wherein the set of dequantizedcoefficients comprises a first subset of dequantized coefficients thatincludes non-zero dequantized coefficients and a second subset ofdequantized coefficients that includes all zero coefficients, wherein anumber of coefficients in the first subset of dequantized coefficientsis equal to the number of allowed non-zero coefficients for the block ofvideo data; apply an inverse low-frequency non-separable transform(LFNST) to the first subset of dequantized coefficients to determine afirst intermediate subset of coefficients; and apply an inverseseparable transform to the first intermediate subset of coefficients andat least a portion of the second subset of coefficients to determine ablock of reconstructed residual values.

Clause 50. A device for decoding video data, the device comprising:means for determining a number of allowed non-zero coefficients for ablock of video data based on a size of the block; means for obtaining aset of dequantized coefficients for the block of video data, wherein theset of dequantized coefficients comprises a first subset of dequantizedcoefficients that includes non-zero dequantized coefficients and asecond subset of dequantized coefficients that includes all zerocoefficients, wherein a number of coefficients in the first subset ofdequantized coefficients is equal to the number of allowed non-zerocoefficients for the block of video data; means for applying an inverselow-frequency non-separable transform (LFNST) to the first subset ofdequantized coefficients to determine a first intermediate subset ofcoefficients; and means for applying an inverse separable transform tothe first intermediate subset of coefficients and at least a portion ofthe second subset of coefficients to determine a block of reconstructedresidual values.

It is to be recognized that depending on the example, certain acts orevents of any of the techniques described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of thetechniques). Moreover, in certain examples, acts or events may beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over as oneor more instructions or code on a computer-readable medium and executedby a hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc, wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore DSPs, general purpose microprocessors, ASICs, FPGAs, or otherequivalent integrated or discrete logic circuitry. Accordingly, theterms “processor” and “processing circuitry,” as used herein may referto any of the foregoing structures or any other structure suitable forimplementation of the techniques described herein. In addition, in someaspects, the functionality described herein may be provided withindedicated hardware and/or software modules configured for encoding anddecoding, or incorporated in a combined codec. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of decoding video data, the methodcomprising: determining a number of allowed non-zero coefficients for ablock of video data based on a size of the block; obtaining a set ofdequantized coefficients for the block of video data, wherein the set ofdequantized coefficients comprises a first subset of dequantizedcoefficients that includes non-zero dequantized coefficients and asecond subset of dequantized coefficients that includes all zerocoefficients determining an intra prediction mode for the block of videodata; based on the intra prediction mode, determining a set of inverseLFNST candidates from a plurality of sets, wherein the plurality of setsincludes 35 sets, and the determining comprising determining an indexindicative of the set of inverse LFNST candidates from the plurality ofsets based on the inter prediction mode, and wherein determining theindex based on the intra prediction mode comprises: for values of theintra prediction mode between 0 and 34, determining the index to beequal to the value of the intra prediction mode; and for values of theintra prediction mode between 35 and 66, determining the index to beequal to 68 minus the value of the intra prediction mode; selecting aninverse LFNST from the determined set of inverse LFNST candidates,applying the inverse low-frequency non-separable transform (LFNST) tothe first subset of dequantized coefficients to determine a firstintermediate subset of coefficients; and applying an inverse separabletransform to the first intermediate subset of coefficients and at leasta portion of the second subset of coefficients to determine a block ofreconstructed residual values.
 2. The method of claim 1, wherein the setof inverse LFNST candidates includes 3 candidates.
 3. The method ofclaim 1, wherein the first subset of dequantized coefficients comprisesM dequantized coefficients, and wherein first intermediate subset ofcoefficients comprises N coefficients, wherein M and N are integervalues.
 4. The method of claim 2, wherein M=16 and N=64.
 5. The methodof claim 2, wherein M=64 and N=64.
 6. The method of claim 2, furthercomprising: determining the number of allowed non-zero coefficientsaccording to the equation NZ=floor((WCM×nTbW×nTbH)/(N×N)), wherein nTbHrepresents a height of the block of reconstructed values, nTbWrepresents a width of the block of reconstructed values, NZ equals thenumber of allowed non-zero coefficients and WCM represents a constantvalue.
 7. The method of claim 6, wherein the first subset of dequantizedcoefficients has NZ coefficients.
 8. The method of claim 6, whereinM=2×NZ and N=64.
 9. The method of claim 1, further comprising: addingthe block of reconstructed residual values to a prediction block to forma reconstructed block; applying one or more filters to the reconstructedblock to determine a filtered reconstructed block; and outputtingdecoded video data that includes the filtered reconstructed block. 10.The method of claim 1, wherein the method is performed as part of avideo encoding process.
 11. A device for decoding video data, the devicecomprising: a memory configured to store video data; one or moreprocessors implemented in circuitry and configured to: determine anumber of allowed non-zero coefficients for a block of video data basedon a size of the block; obtain a set of dequantized coefficients for theblock of video data, wherein the set of dequantized coefficientscomprises a first subset of dequantized coefficients that includesnon-zero dequantized coefficients and a second subset of dequantizedcoefficients that includes all zero coefficients; determine an intraprediction mode for the block of video data; based on the intraprediction mode, determine a set of inverse LFNST candidates from aplurality of sets, wherein the plurality of sets includes 35 sets,wherein to determine the set, the one or more processors is configuredto determine an index indicative of the set of inverse LFNST candidatesfrom the plurality of sets based on the inter prediction mode, andwherein to determine the index based on the intra prediction mode, theone or more processors is configured to: for values of the intraprediction mode between 0 and 34, determine the index to be equal to thevalue of the intra prediction mode; and for values of the intraprediction mode between 35 and 66, determine the index to be equal to 68minus the value of the intra prediction mode; select an inverse LFNSTfrom the determined set of inverse LFNST candidates; apply the inverselow-frequency non-separable transform (LFNST) to the first subset ofdequantized coefficients to determine a first intermediate subset ofcoefficients; and apply an inverse separable transform to the firstintermediate subset of coefficients and at least a portion of the secondsubset of coefficients to determine a block of reconstructed residualvalues.
 12. The device of claim 11, wherein the set of inverse LFNSTcandidates includes 3 candidates.
 13. The device of claim 11, whereinthe first subset of dequantized coefficients comprises M dequantizedcoefficients, and wherein first intermediate subset of coefficientscomprises N coefficients, wherein M and N are integer values.
 14. Thedevice of claim 13, wherein M=16 and N=64.
 15. The device of claim 13,wherein M=64 and N=64.
 16. The device of claim 13, wherein the one ormore processors are further configured to: determine the number ofallowed non-zero coefficients according to the equationNZ=floor((WCM×nTbW×nTbH)/(N×N)), wherein nTbH represents a height of theblock of reconstructed values, nTbW represents a width of the block ofreconstructed values, NZ equals the number of allowed non-zerocoefficients and WCM represents a constant value.
 17. The device ofclaim 13, wherein the first subset of dequantized coefficients has NZcoefficients.
 18. The device of claim 13, wherein M=2×NZ and N=64. 19.The device of claim 11, wherein the one or more processors are furtherconfigured to: add the block of reconstructed residual values to aprediction block to form a reconstructed block; apply one or morefilters to the reconstructed block to determine a filtered reconstructedblock; and output decoded video data that includes the filteredreconstructed block.
 20. The device of claim 11, wherein the devicecomprises a wireless communication device, further comprising a receiverconfigured to receive encoded video data.
 21. The device of claim 20,wherein the wireless communication device comprises a telephone handsetand wherein the receiver is configured to demodulate, according to awireless communication standard, a signal comprising the encoded videodata.
 22. The device of claim 11, further comprising: a displayconfigured to display decoded video data.
 23. The device of claim 11,wherein the device comprises one or more of a camera, a computer, amobile device, a broadcast receiver device, or a set-top box.
 24. Thedevice of claim 11, further comprising: a camera configured to capturevideo data.
 25. The device of claim 11, wherein the device comprises awireless communication device, further comprising a transmitterconfigured to transmit encoded video data.
 26. The device of claim 25,wherein the wireless communication device comprises a telephone handsetand wherein the transmitter is configured to modulate, according to awireless communication standard, a signal comprising the encoded videodata.
 27. A non-transitory computer-readable storage medium storinginstructions that when executed by one or more processors cause the oneor more processors to: determine a number of allowed non-zerocoefficients for a block of video data based on a size of the block;obtain a set of dequantized coefficients for the block of video data,wherein the set of dequantized coefficients comprises a first subset ofdequantized coefficients that includes non-zero dequantized coefficientsand a second subset of dequantized coefficients that includes all zerocoefficients; determine an intra prediction mode for the block of videodata; based on the intra prediction mode, determine a set of inverseLFNST candidates from a plurality of sets, wherein the plurality of setsincludes 35 sets, wherein to determine the set, the instructions causethe one or more processors to determine an index indicative of the setof inverse LFNST candidates from the plurality of sets based on theinter prediction mode, and wherein to determine the index based on theintra prediction mode, the instructions cause the one or more processorsto: for values of the intra prediction mode between 0 and 34, determinethe index to be equal to the value of the intra prediction mode; and forvalues of the intra prediction mode between 35 and 66, determine theindex to be equal to 68 minus the value of the intra prediction mode;select an inverse LFNST from the determined set of inverse LFNSTcandidates; apply the inverse low-frequency non-separable transform(LFNST) to the first subset of dequantized coefficients to determine afirst intermediate subset of coefficients; and apply an inverseseparable transform to the first intermediate subset of coefficients andat least a portion of the second subset of coefficients to determine ablock of reconstructed residual values.
 28. A device for decoding videodata, the device comprising: means for determining a number of allowednon-zero coefficients for a block of video data based on a size of theblock; means for obtaining a set of dequantized coefficients for theblock of video data, wherein the set of dequantized coefficientscomprises a first subset of dequantized coefficients that includesnon-zero dequantized coefficients and a second subset of dequantizedcoefficients that includes all zero coefficients; means for determiningan intra prediction mode for the block of video data; means fordetermining, based on the intra prediction mode, a set of inverse LFNSTcandidates from a plurality of sets, wherein the plurality of setsincludes 35 sets, wherein the means for determining the set of inverseLFNST candidates comprises means for determining an index indicative ofthe set of inverse LFNST candidates from the plurality of sets based onthe inter prediction mode, and the means for determining the indexcomprising means for determining the index to be equal to: for values ofthe intra prediction mode between 0 and 34, the value of the intraprediction mode; and for values of the intra prediction mode between 35and 66, 68 minus the value of the intra prediction mode; means forselecting an inverse LFNST from the determined set of inverse LFNSTcandidates; means for applying the inverse low-frequency non-separabletransform (LFNST) to the first subset of dequantized coefficients todetermine a first intermediate subset of coefficients; and means forapplying an inverse separable transform to the first intermediate subsetof coefficients and at least a portion of the second subset ofcoefficients to determine a block of reconstructed residual values.